Some are, but there's not as much of a market for them.
Think about it. If you want the best domain for [your brand], what would it be? [your brand].com, obviously.
If you can't get that, what do you do?
- Rebrand.
- Add a keyword to the SLD.
- Get another TLD.
Presuppose someone opts to go for option 3. Then there's still a bunch of competition: .ai, .io, etc. and the chances of you getting it right with he .net are slim.
But even if they want the .net as the second alternative. There's a chance that they didn't get the .com because of budget-constraints. So now you have to price your domain competitively, so they don't decide to go for the .com anyway.
But anyone worth their salt knows that they need the .com. Take gandi.net, they've been around for 25 years and people still think of them as a bush league company.
I've said it before and I'll say it again, no one cares about the silver medalist.