I don't.

The overwhelming majority of road bike buyers have no clue what the UCI is, let alone what it has to do with their bikes. They don't give a crap about racing.

Is the guy who wants to ride to work going to look for a "UCI compliant" sticker? How about the guy who wants to ride on the beach with his girlfriend?

The only people who really care are the ones who are racing. And since it is easy for the manufacturers to accommodate the racers, without short-changing everyone else, and since the racers are likely to buy lots of bikes and swag, they might as well make the bikes UCI compliant.

Or, to put it another way: If the manufacturers believed recumbents could sell, they'd sell them. Marketing their way around the UCI would be a trivial challenge.

We see this with folding bikes, which are definitely not UCI compliant, but are a growing segment of the market regardless.