How so?
Do the liches not lich enough or the dragons not breathe hard enough?
When it comes to encounters, there's a spectrum between having hard rules and having a dynamic set of encounters.
If every enemy had to have this much HP and damage output by this level and can only be faced in this order, the players could easily predict that these goblins are stronger than those goblins because those goblins were encountered at level 1.
In contrast, too dynamic and the encounter designs can be pure chaos with a DM not having any real sense of what is right and wrong. How do you compare a 1hp, 1AC creature with PWK against a creature that has over a million HP but does 1 damage every 100 turns?
WoTC is more on the dynamic spectrum than static spectrum. They don't want the game to feel mechanical in that everything the party faces is catered to them and the DM isn't allowed to put ancient dragons in their world until the players themselves reach tier 4. Of course, the dynamics might not be to everyone's liking but calling it bad design is different from calling it a design you in particular don't like.
I enjoy that a lich, a vampire, and a mummy lord have distinct ways to fight. They didn't have to make them like so, but they did and it allows me as a DM to continuously switch up the combats against being meatstick matches or counterspell contests.