Shh don't tell Uli Behringer about it /s
I think it is because "just making same thing" is frowned upon and also industry cares to not eat itself.
For example take Digitakt and Digitone. It could easily be combined into one device "it is just code" and CPUs got faster since release of those but they don't, because they know having those "blind spots" in each of their devices (that other of their devices fit) is good for sales. Got a Digitone ? Want Drum machine to it ? Well, Digitakt got same interface you already know. Even their newest device Syntakt omits a lot of FM stuff to not step on Digitone niche.
Also there is plenty all-in-ones with more capability, all the way to monsters like MASCHINE+, MPC X or Force. People buy OP-1 coz they want to work within the limits and think that will make better music, not because it can do something unique anything else can't.
Which is understandable, given myriad options it's easy to get lost in tinkering with them instead of focusing on creation.