Yeah, I've often wondered the same thing too.
But the term eMTB spans a very broad spectrum. Different motors have different torque outputs, riders have different riding techniques or goals, battery capacity varies as well as trail topography - they all change the gameplay entirely. It all depends on the bike and how and where it's being used.
Riding specifically for gaining fitness or extracting maximum mileage out of one battery charge - would require running on the lowest or ECO mode as much as possible. With the extra 10 or so kilos on eMTBs, that would get close to mimicking riding a manual bike. With a restricted Class-1 eMTB, a 10 speed cassette (or even an 11speed) has benefits because limitations of human energy and cadence management is still very much present in that equation.
On the other hand a non-restricted Class-3 eMTB with 500W+ motors and large battery capacity can be shifted electrically (by just scrolling up and down the power-modes), and so minute increments in cassette cog sizes is not that important - as well as a high number of cogs for that matter. A 9 speed or even a 7 speed with a thicker chain might make better sense in that configuration.
The market of course represents both extremes and everything else in between. I guess manufacturers have to cater for various user-preferences and eventualities to make their products adaptable.
I've got a 10-speed 11-36T cassette in mine and even on steep climbs I've never had the need to use the 36T cog, and I find myself always shifting at least two clicks most of the time to get to the right gearing. I was thinking of upgrading to an 11-42T cassette until I saw the weight of those things. So I'm now considering to go down to a lighter 11-34T cassette - but I bet the moment I do that, I'll end up on a trail where I would need the 36T!
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