Because they have been in the line just as long as you have, and they aren't leaving it. They don't know if you're really using the bathroom or are thirsty or just want to walk around a little bit or whatever. All they know is that they've been waiting in a long, long line for a long, long time, and you're sliding past them. If it's a 10 minute wait, no one probably minds. If it's a 40 minute wait, some people will get annoyed. But if they've been in line for an hour and still have an hour to go, and then someone -- anyone, no matter the reason -- wants to get past them, it's aggravating. It's expecting other people to be considerate when you aren't being considerate -- you're expecting them to put your needs above theirs.
I'm not sure how I'm expecting to put my needs above theirs? I'm hardly being inconsiderate by needing to go to the toilet, unfortunately we all have bodily functions that regularly need attending. This is especially true of children who, when they have to go to the toilet, they absolutely have to go now! More so the younger ones. While usually rides with longer lines aren't for the under fives, I can think of several (Frozen Ever After and Peter Pan which regular have long waits) If I've been waiting for two hours with an hour still to go and I'm in line with a child who has to go to the toilet, I'm not waiting another three hours plus the two I've already waited. It's unreasonable to expect a person, especially with young children to wait in a 5 hour line when the actual ride is only 2-3 hours. If this happened to me, I'd speak with a CM and explain the situation. I would be happy to return to the line and wait another hour if I had been in line 2 hours and the wait was 3. I'm NOT going to the back of the line. If another guest had an issue with that, I'm sorry but it wouldn't bother me. I'm not cutting the line, I'm attending the needs of a child and I've waited in the line as long as you. I assure you, I don't WANT to do the toilet dash with a toddler, it's life with a child.
But, you ask, how could anyone object when my child needs to use the potty? They could say if your kid can't handle a long wait, that's the risk you take. They could say you knew how long the line was when you got into it, and maybe you should have thought ahead in greater detail. They could say I'm hot and sweaty and bored and I made sure my kid went to the bathroom before we got in line, and if you had done that, then maybe I'd be that much closer to getting on the ride. I mean, you could easily flip the question, couldn't you? Why would anyone expect someone who has been waiting in a line two hours to let someone leave it and then come back the spot they left? Part of the WDW social contract is that you wait in lines -- why should you get to violate it?
In my experience, you can take a child to the bathroom and within ten minutes, they absolutely MUST go to the toilet again. Children don't have the same cognitive functions as an adult, nor do they have the physical control over their bladder. I would prefer to take my child to the toilet as opposed to having people step over a puddle. I personally would never have a problem with a parent/caregiver leaving the line with a child to use the bathroom. If someone does have a problem, I think that says something about them. Also, if I've been in line, leave the line and return, no one behind me has an additional wait. They will get on the ride exactly the same time they would have if I hadn't left the line.
And while I'm sure you're a nice person with a great avatar, all the people behind you who didn't see you leave know is that you're moving past them in a long, long line. They don't know if it was a bathroom break of Joey holding your spot while you did the Safari and watched the bug show -- all they know is you and your party are moving past them in a long line. Of course they're aggravated; most people would be.
I personally wouldn't be annoyed, especially if I could see it was a parent with a young child. Why would anyone be annoyed at a parent taking care of their child, especially for something like a toilet break. I would prefer they leave then having to step over the puddle or hear (or smell!) a very upset child who has soiled themselves. Most people aren't cutting lines. Yes, you will always get some but the majority of people are doing the right thing. I personally can't see the issue.
All of that said, my experience has been that most people are perfectly fine letting someone use the bathroom and reclaim their spot. Most people -- sigh -- are even accepting of the "catching up with whoever" activity. But that's only because most people are being nice. When you leave the line for whatever reason, you're depending/expecting/demanding other people be nice to you. Most of the time they will be, but it's hard to really condemn those who choose otherwise. Especially after a long, long wait in a long, long line.
I'm not really expecting people to be nice. I'm expecting people to be fair, which is exactly what I always am. I'm not going to stress about lines, or someone going to the toilet and even line cutting to be honest. I have a very stressful life at home and my job, I'm not going to stress out because someone has to leave the line and use the potty.
Also, I just want to point out that this is entirely hypothetical. I don't travel with children but I know what it's like to care for them so I would never begrudge a parent for doing what they have to do. I cannot begin to imagine the stress of doing WDW with children. It's hard enough for us with all adults. My hats off to anyone who brings children to the parks and if they've been in line to meet Cinderella for two hours and needs to use the toilet, I don't have an issue with them leaving and coming back. No one wants to meet a Princess with a stain on their pants!
And thank you for the compliment on my avatar. I love Giselle, I wish she met in the Parks!