Here’s a bit more of a technically oriented explanation. A power meter measures how much you bend the cranks. Every crankarm is different, and it will bend a different amount under the same force. And there are quite a few crankarms in service. If you’re making power meters for standard Shimano crankarms, you probably have to do at least 3 models (the major 3 road groups), but then you have the gravel groups (also 3 slightly different crankarms). You might decide you want to forget about SRAM users, since they already have Quarq’s PMs, and you might decide to forget about Campagnolo since nobody uses Campagnolo anymore. Nevertheless, you might want to think about those guys, and there are still third party crankarms to think about.
Exercise bikes don’t use standard cycling cranks. So, a power meter manufacturer would need to go and see what types of crankarms are mounted to those bikes, and then figure out a) if it could be worth their business, and b) can they physically mount a sensor pod on the arms? To the second point, in cycling, a lot of crankarms have this nice flat surface to which you can bond the strain gauge pod. The crankarms on exercise bikes might not. To the first point, I don’t know how many models of crank are typically found, but for each one, a manufacturer would have to go and determine how much a known weight bends the arm. So it’s not just bonding the sensor pod onto the arm, there’s some empirical testing. For each model.
And I’m probably oversimplifying a bit, but I suspect this is pretty close to the truth. You avoid all that with pedal power meters. I mean, you could email Stages or 4iiii and see if they would consider this.