I miss that too!
As an aside, there is a lovely irony in your asking why Disney hates trees and grass so much in response to vintage pictures of the plazas in front of SSE and The Land. Disney has removed trees at SSE. And added them at The Land - the one place where nature is presented more abstracted. Here they lessened the design by too much greenery, by turning the entrance plaza into a veritable jungle. Where they needed trees, they cut them. Where they did not, they added them.
I think much of it is just a matter of simply 'imagineering' for the sake of it. If you think a place is perfectly well designed, well, then there is little for you to add, is there. No work, no opportunity for you to leave your mark over the previous dog's mark.
There is no honour in just mowing the grass, in upkeeping a place. No, you want to rethink it, re-do it, be an EPCOT designer too, have your own little reception and grand (re)opening, fill your resume, have your turn at the helm. It is all perfectly understandabe, but the work is often pointless and the results comical.