They have lots of options to make it harder.

They could put the commercials directly into the stream and disallow fast-forward/start at time commands.

They could run drm and change the key/method up so often blockers would need to change on a daily basis.

They could shrink the content into 1/4 screen and run the add at 3/4

Competition would take their lunch eventually.

There's a balance between keeping your customers happy and making your advertisers happy.

I think its simply that its a completely different medium. People dont want to be up to date with a video, and they also aren't fine with missing a part of the video.

With a stream, you simply never get the parts that were overtaken with ads.