Three parks are getting worked on--DS, AK, and MK. DS is still in baby stages, but I for one can hardly wait for a huge Star Wars section!
Sure, NOW three parks are getting worked on.
The last ride to open at EPCOT just turned 10. There are many threads on the dis complaining about how EPCOT and DHS seem gutted. There just isn't as much to do there as there used to be. At EPCOT alone Body Wars has closed, Captain EO has closed, Maelstrom has closed, they took a lot of the live performers out of WS, the Imagination Pavilion is laughably bad. They recently closed Maelstrom to do a Frozen overlay that is going to take around 18 months which seems ridiculously excessive given they aren't building an entire ride.
Recently DHS has closed the Backlot Tour closed. The last ride built was TSMM in 2008 and the last attraction opened was Disney Jr 4 years ago in 2011.
Before Under the Sea opened in New Fantasyland in 2012 the last new ride that opened was Aladdin in 2001. They recycled Alien Encounter to turn it into Stitch in 2004, Mickeys Philharmagic opened in 2003, and Monsters Inc opened in 2007. Everything else was new parades, dance parties, or character meet and greets.
If they started to do much more work you would really have people complaining. They can't do everything all at once.