Because if they didn't Average Bob would feel like they're missing out.

Some people don't care about 4K. Other people think they care about 4K but don't really know the difference.

"4K streaming" is such a joke

They compress and screw up the video quality so much that even calling it 1080p is a stretch in my opinion

Most people have houses/screens too small for 4K to be visible anyway, though, so I guess it's easy to get away with providing such a crappy service