Apparently, they see insufficient benefit in higher real resolution. The question why this is the case would be an interesting one, but I couldn't answer it.
I don't think they don't see the benefit of higher real resolution. I just think that they know that they can't get it easily (and cheaply) from higher resolution sensors.
Epson has been using CCD sensors with 40.000*+ pixels (per channel) for two decades now. The sensors are not the problem. On the other hand, lenses inside Epson desktop scanners are a fraction of a size of those in pro desktop scanners made 30 years. And I'm talking about lenses in "xy-stitch" scanners where lens only needs to cover small part of the scanning bed!
This is your typical consumer scanner lens (left) that I took from Canoscan 9000f - Canons equivalent to Epson 4990 vs. Componon-S 100/5.6 enlarger lens (right):
Here the same Componon-S 100/5.6 (left) and lens from Scitex EverSmart pro desktop scanner (right):
Remember, this lens from Eversmart needs to cover only a fraction of the area, Canon lens needs to cover the entire width of the scanning bed. I think one can safely assume where the "softness" in consumer desktop scanners comes from even if we don't count or worry about all the individual pixels of the sensor. There are plenty of them, there is not much need for additional software interpolation (which is still used in some modes to match vertical vs. horizontal resolution).
You'd only need even better sensor in a scanner if you'd have much better lens. Which means bigger size, higher weight, better build, more expensive...