Why is it less satisfying? Does it feel like "cheating" because you're just making things up instead of sticking to what's written down? Or maybe not cheating, but arbitrary?
Well, in a nut shell, yes.
a) As a GM, I don't trust myself to run a scenario fairly between the players and an NPC if the NPC has the power of plot.
b) As a player, I don't trust the GM to run a scenario fairly between the players and an NPC if the NPC has the power of plot.
c) As a designer, I see that many of the worst excesses of 1e/2e were owed to the fact that NPCs were different the than PCs. For example, NPCs could create magical items, traps, or objects with arbitrary abilities because 'plot'. PCs in the same situation had to leap through fantastically high hoops. It was a methodology that excused keeping DMs fully in control of the 'plot' and the sort of solutions that were acceptable.
But I like the fact that I can make simpler monsters more interesting myself, on a case-by-case basis.
Yes, but this is not a feature. You can make monsters more interesting whether they are simple, complex, interesting, or uninteresting. The point is that I pay the professionals 'big bucks' so that I don't have to do so myself. Equally, a stat block that is complicated I can always cut down to something simpler if I need to. For example, I regularly run 3e monsters without worrying about all the skill points and whether or not I've got every single stat exactly right.
More to the point, while you or I may well be able to do fine without a rule book at all, drawing upon our vast experience to create scenarios more complicated than 'Enter the scenario', 'Kill the monster', 'Take the monsters stuff', and 'Leave the scenario', I'm not all convinced that this sort of presentation encourages someone with less experience to do or imagine anything else. This sort of presentation suggests to me that monsters are for killing in straight forward hack and slash combat and I don't see how it would suggest anything else. That isn't all bad, but it's not all good either.
Beyond the lack of inherent creativity in the monster and the fact that that burden is shifted on to the GM, I'm beginning to see a trend toward one dimensional high level monsters. Coming from my 1e/3e background, this worries me that there is insufficient play testing at higher levels. Against creative players with high level PC's, it's often the case that the PC's' are able to leverage absolute advantages against particular strategies. Generally when designing a monster meant for high level play, I take care to give the monster alternative strategies beyond its core strategy. When looking at this monster or even more so at the Tarrasque, I'm looking at a monster with few options if its core strategy doesn't work. What does it do against PC's that have both ranged attacks and move faster than it? Even so much as giving the Tarrasque a borrowing speed, something that doesn't complicate the monster much, at least gives it options that reduce the chance that the Tarrasque is reduced to being a chump. While 5e may be reducing the options of high level PCs somewhat, I doubt they are so reduced in practice that against a creative player won't at times make them trivial.
This creates a problem. A DM encountering the problem of the Tarrasque going down like a chump is going to be tempted to create (or more transparently argue for) a borrowing speed on the fly - particularly if they didn't foresee the problem with the stat block ahead of time. But this is in fact metagaming, and quite rightly can be perceived by the player as cheating/ruling in favor of the NPCs and that undermines trust. Even more so, if the DM gets creative off stage with the powers of a bone devil, when faced with a similar situation you are risking ruling that the monster has power of plot on stage. (See some 1e and almost all 2e module writing.) That sort of problem I think was very much tied to the simple stat block doesn't let the monster do things it reasonably ought to be able to do, but on the other hand if you can invent what the monster can do, does it have any real restrictions other than what the DM wants?