Yeah, that's one of the moats some have but booking.com has not: you don't just pop up a hotel from nothing, and the hypothetical failure mode of complete fakes appearing out of thin air is not only solved by payment on site but also by the local hotel owner community having a financial interest in policing their turf.

Or not, could be a fun plot for some speculative fiction: on a cooperative post-booking.com platform, hotel owners in a remote valley discover that they can earn quite nicely by offering "rescue nights" to guest who fell for a series of fake listings they keep up, untraceably because the fakes lack a payment channel to follow. Add some Bates vibes to the one hotelier who refuses to take part in the scheme for multi-layered tension.

A cooperative booking organization would have strong incentive to keep their members in the straight & narrow. It is a cooperation problem and institutional enforcement is a proven recipe for ensuring that.

Not sure where the "random portal" creeps in. A portal underwritten by all the hoteliers in a country / region is abouy as much assurance you can get.