You also answered you own question there. They have so many projects going on a once whereas Universal has just a few at most so progress is made at a faster pace.
This feels kind of like saying "Why are bathrooms cleaner at Universal?" and someone coming back with "Do you know how many bathrooms Disney has across property compared to Universal? Do you realize how much more work it is to clean all those Disney bathrooms?!"
I'm not saying that Universal has cleaner bathrooms than Disney but the comparison makes the validity of the argument that this should matter more obviously flawed. A larger operation should by simple scale, have more people to manage projects.
It's not like Disney suddenly had a growth spurt in developed land over the last decade that makes their construction/project needs suddenly a surprise. If Disney is genuinley finding themselves stretched thin, they could have avoided some of it by breaking ground on the 3rd theater half a decade or more earlier back when it was apparent to a first time visitor from Tanzania that they had a capacity problem with this attraction in Epcot - just like TSMM at Hollywood Studios. It isn't breaking news to anyone visiting the parks so why act like management had no choice but to approve these attraction expansions in the middle of
real new development across property?
Furthermore, there are a limited number of construction resources available in Central Florida but if that were an issue, we wouldn't see other people building at the pace they are.
Disney in recent times has also moved much more heavily to outsourcing aspects of attractions that were previously strictly in-house so that argument on volume doesn't really hold much water, with a lot of what might have previously fallen under Imagineering, either.
The fact of the matter is, Disney moves at the pace that they do because they choose to. Do they need to explain or justify that to us? Probably not.
Do we have to like it? Absolutely not.