Because stuff like the Bullet Cluster exists. Dark matter is in fact the simplest answer.
Light being the same speed irrespective of observer is weird. Velocity dilating length and time is weird. Not having a preferential observation point is weird. Not needing a medium for transmission is weird. Not being able to agree on simultaneity is weird. etc.
Aether wasn't just something that a bunch of dullards clung to. You have to abandon some very long held common-sense understanding when you give it up.
Aether: observations show lack of aether so we update the theory that makes aether unnecessary.
Dark matter: observations show lack of observable matter so we keep hunting for decades locating it. The aether analogue would have been to continue to look for dark aether. Dark matter is more like god of gaps. We can 'darkify' any theory that does not fit empirical evidence.
It's hard to tell now which leans more towards the more correct theory.
For example "angry gods" was never a simple theory, one needs only to read some fan fiction to understand that theology gets complicated fast. Instead "angry gods" is a simplified summary of a very complicated theory about metaphysical hierarchy, creation, agency, the meaning of divinity, and cause and effect, etc etc.