High speed receivers are much more complex than transmitters. Just shoving some bits down the line with a bit of emphasis filter training is not that tricky. Tuning a receiving emphasis filter while simultaneously recovering the clock and performing error correction requires many more transistors, which uses more power and costs more to produce. HDMI receiver chips on digikey are 2-10x more expensive than transmitter chips.

Beyond the technical reasons, there is the fact that media companies have a vested interest in not having video input being prolific. They have a long history of fighting technologies such as VCRs. In the shadowy internet era you can let your imagination run wild with what motivations they set up in what industries to make HDMI receivers less prolific.

Also, for those talking about HDMI over Ethernet: be careful. HDMI is actually quite high bandwidth. There’s a reason the cables stop working after a few feet and it isn’t because they’re poorly designed or manufactured (though that’s surely the case for crap you get on amazon). Cat 1 million isn’t magic. It’s still beholden to Shannon’s limit in copper twisted pair. You can’t shove 25 GbE over any copper longer than a few meters of TwinAx. There are plenty of solutions on the market that are too good to be true and simply won’t work at the pixel clocks they advertise, yet will happily take hundreds of your dollars and weeks of your time.