There are wheelchairs outside Pirates - or there should be - for transfer purposes. Obviously a disabled person can't be made to walk through the line, so anywhere you've got to park your
ECV as far as I know should have them.
You are correct - any of the attractions that are not ECV accessible do have attraction wheelchairs that can be used. Occasionally, they are all in use, and you may have to wait for one to become available.
Some attractions may not have a long wait, but the distance walked may be quite long. Pirates (as the OP found) is one of the attractions with a long walk - more than 1/4 mile. Some other ones are Soarin, Kilimanjaro Safari, Bugs Life. Unlike Pirates, those do have lines that are ECV accessible, but some people park the ECV and walk.
If you are asked to park the ECV for an attraction and dont think you can walk, make it clear that you cant walk in the line and will need a wheelchair.
The Pirates line is not ECV accessible and they actually also ask guests to park their personal wheelchairs for Pirates.
The reason they want guests to leave their personal wheelchair with the strollers is that it is difficult and time consuming to get a wheelchair from the boarding area to the unload area. The unload area is actually in a different building and one level down compared to the boarding area. If someone is using an attraction wheelchair, it doesnt matter how long it takes to get that wheelchair to the unload area because there will be another attraction wheelchair waiting there that the guest can use to leave.
If a guest uses their own wheelchair to the boarding area, the CMs have to get that particular wheelchair to the unload area before the guest gets there. In the past, that has sometimes been a challenge and a few people have posted that they arrived at unload before their own wheelchair arrived.
That is why they want guests to park their wheelchairs with the strollers and use an attraction wheelchair if they cant walk.
At Pirates at MK in WDW, there is a moving ramp to get back to ground level from the lower level where the unload area is. As was already mentioned, there is an elevator to the left of the moving ramp, down a small hallway.