Sure it’s a valid musical form. Not a very good one, for me, but still valid.
It’s just one I happen to dislike immensely. While I can appreciate the political messages, especially in the earlier days of rap, the simplistic musical delivery system for those messages, is boring, cliché, repetitive, poorly played, IMO. In other words, it has a very limited musical language.
My criteria for what I consider good musical forms are: very high level of musicianship, musical complexity, deep and broad emotional and intellectual content, avoidance of verse>chorus>bridge song structure, no need for simple hooks.
In other words, almost the complete opposite of rap.
And yes, I have listened to rap many times, in all eras. Every time I mention on music forums why I don’t like it, someone will invariably mention rap that they believe fits some of my criteria. Having an open mind, I will listen to it, and always come away disappointed. It is always simple, repetitive, lacks any real musicianship, etc.
Just to sum up, my dislike for rap is purely based on musical content and merit. No other criteria at all enters my judgement. I also put pop, mainstream rock, punk, disco, country at just a slightly higher level than rap, again due to those criteria I mentioned above.

