I'm contemplating about getting an electric assist MTB but as much as I can see all (or at least majority) of e-bikes are equipped with 1x transmission.
Is there a reason why there are no e-bikes with more than 1x drive-train?
Well, there are a couple of reasons most MTB ebikes are 1x models:
Modern motors on ebikes provide enough of a kick to get you up a steep hill as well as enough speed to go 50kmh on flats. Even a basic 750 watt motor is stronger and faster than almost any rider's legs. I’m not sure what more gears would give you except more complexity and weight.
A modern wide-range, mountain derailleur gives you more than enough capacity and given the motor assistance, you don’t need to be at the optimal cadence that closer spacing would give you
Many higher end ebikes have bottom bracket motors such as the Bosch or Yamaha systems, which don't allow for front derailleurs (the Bosch for example has internal gearing - a big advantage of which is that the chainwheel is very small and allows for greater clearance with a lower bottom bracket height)
Many battery packs are located behind or near the seat post. This makes the placement of a front derailleur difficult or impossible.
Some ebikes have throttles which makes the placement of shifters on both sides more complex.
Of course, it's really easy for you to add your own rear hub motor to an existing 3x10 MTB if you really want one. But the cheaper vendors are going 1x to save money and the expensive vendors are using bottom bracket motors which aren't FD compatible.
1) Low-end fat ebike (Sondors fatbike, around US$700-1000, rear hubmotor). The battery compartment takes up the entire main triangle, which means you can't fit a front derailleur without some cutting and sawing. Not having a front derailleur also no doubt saves some money. Note that this is the fat Sondors but he regular Sondors has the same design.
2) High-end Specialized MTB (around US$3000; Bosch mid-drive). The Bosch bottom bracket motor means that a front derailleur is impossible to fit as the motor/bottom bracket has internal gearing.