Roughly speaking, it is because it does not follow the Unix philosophy and proposes to do several tasks making the code very complex and therefore more susceptible to bugs.
The joke being "systemd does everything poorly". First heard someone say this about X and Wayland. People were saying Wayland violated Unix philosophy and the speaker said "name one thing X does well" lol.
The linux kernel unfortunately does not follow unix philosophy.
It would be better in various ways if the linux kernel used a micro kernel architecture following the unix philosophy, something Torwalds acknowledged in the past.
Philosophical ideas being lost does not mean they're outdated.
How old in the past did Linus acknowledge it? my source says he dislikes/disliked microkernel, it dates to 2001 ,if you have a more recent source proving that he no longer thinks it I'll look at it
Unfortunately I can't remember the timestamp, but it's right around when he starts speaking about when the MINIX creator bashed him, IIRC
(not to bash on you, but this implies the point you're making, that Linux shouldn't have a monolithic kernel is 30 years old,)
As opposed to which other tools that respect Unix philosophy. Philosophy which I might add is severely outdated. That could have been a thing when you have simple command line interfaces but pretty much every application today violates that philosophy and nothing of value was lost.