... *Game designer hat on*
you know that bosses generally have limited amounts of attacks for a
reason, right? Firsto f all there's the logistical issues - every new attack needs to have a specific behavior programmed and animated - and if it affects anything else then
those need to be programmed and animated too. That takes loads of time on it's own, let alone the time ti would take to
design all of the attacks. It's not just a mattero of saying, "yeah, this would look cool", we also need to consider how the player is going to react to it, how it's going to affect thep layer and how it will balance the fight,
and how well it interacts with the environment you're given, the player's abilities, etc. Basically, each new attack requires a lot of thought with lots of componants, and therefor lots of things that can go wrong.
Setting all that aside, there's also a very good reason to have limited attacks fro the players perspective too. The boss battle is generally intended to be an test of sorts - they test your skill level, your knowledge of the mechanics, etc. For this reason early bosses tend to have a strict gimmick - they need to test how you've learned a single mechanic.
Later bosses are tests of more general skill, but they follow the same general pattern. Most bosses are essentially beaten by the ability to recognize, predict, and react accordingly. In other words, the player
must be able to recognize what they are doing in order to avoid it. If they can't predict the attack it takes away the sense of agency and control when they're hit, and it becomes frustrating. So the designers use a limited number of attacks, so that the player will be able to see, recognize, and react to the them all using the skill sets the game has taught them.
It's not feasible from a designers perspective to include lots of attacks, cool as we all know it would be, nor is it enticing from the player's perspective. The only way for it to work is to have what is essentially an interactive cut scene, and while that does it for some others just find it aggravating and pointless.