OpenAI and google are too scared of music industry lawyers to tackle this. Internally they without a doubt have models that would crush these startups over night if they chose to release them.
Thus far AI has only been used to create fan fiction clips that generate free marketing for legacy IP on TikTok. And the rights holders know that if AI gets good enough to make feature length movies then they'll be able to aggressively use various legal mechanisms to take the videos off major sites and pursue the creators. Long term it could potentially lower internal production costs by getting rid of actors & writers.
Music is very different. The production cost is already zero, and people generating their own Taylor Swift songs is a real competitive threat to Spotify etc.
Not qoez:
You have to balance market opportunities with the risk of reputational damage and litigation risk.
Video will probably make a lot more money than audio, so you are willing to take a bigger risk. Additionally, at least for Google there exists a strong synergy between their video generation models and YouTube, which makes it even more sensible for Google to make video models available to the public despite these risks.