Honestly, Toy Story 3 was the perfect ending and everything after that was going to feel like a unnecessary sequel. But considering that, unlike a certain other movie, Lightyear didn't exactly wow people makes it hard to be all that excited for another Toy Story.

After how Toy Story 4 sucked so much it's no suprise people just want that series to just stay as a part of their childhoods and never come back.
As for Shrek, I imagine it's because people want to see what the Dreamworks that made Puss in Boots The Last Wish and The Bad Guys can do with it. Next thing is speritual sequels to How to Train Your Dragon and Kung Fu Panda, which got a series last year, with Jack Black and all.