Which gets into ludicrous territory pretty quick, since a dungeon becomes "you open a door and see 30 orcs playing cards". I have to roll for all these losers, who have suck AC, suck damage, and then track all their friggin hit points. Its a busywork treadmill to achieve the same thing 4E did better with minions.
Minions in 4E generally had suck AC and damage too, you just didn't have to track HP. You could do the exact same thing in 5E by marking them like the "2 Hit" minions I saw many people using in 4E. Ignore the damage, as each creature dies in 2 hits (or one failed save for half damage).
Or you could design better encounters. Rather than having them sitting around playing cards, they could actually have fortifications to hide behind (while firing crossbows), scouts/guards to provide advance warning, traps to hem the players in and reduce HP by attrition. I did this in 1E with kobolds to a 26th level magic user (who'd gotten a bit too obnoxious); he never did manage to kill them all before he had to teleport to safety, and this was LONG before bounded accuracy.
Monsters in 5E are just sad, boring wusses PC's are meant to steamroll over in a round. The exception being casters (naturally...) which means I need another damn book handy, where 4E had the courtesy of keeping all the crap I needed to run a monster on the monster entry. It's like they designed for a party of champions all wielding non-proficient improvised weapons and stat arrays of straight 10-12's... God forbid you actually hand out magic items or let people use feats (or roll for stats).
The problem isn't the monsters... it's the lame Encounter Building guidelines that are specifically designed to be "easy mode." I used 2 Adult White Dragons on a "Hard" Difficulty that was very nearly a TPK, while the same group would handily defeat "Deadly" Difficulty x2 (or sometimes x3) of Giants and such. I knew this would be the case, because the dragon encounter was meant to be tough, while the Giants were not (regardless of what the guidelines said).
As for spellcasting monsters, this is a matter of preference. I would rather have more space to give out information about the monster (such as ecology, history, etc.) or to simply have more monsters. Page count is an important thing in publishing, and putting repetitive information down is going to bloat that to the dismay of most customers. The only thing I would really have liked to see is a page reference, so you can quickly flip to the appropriate page. Note cards are a good aid when using spellcasting monsters, or just reviewing them before the session. Even better would be a tablet and a PDF of the basic rules you have bookmarked, since most monster spells are there.