Please ignore current supply chain shortage issues as that is temporary (hopefully) and EV's were expensive prior to covid as well
I _suspect_ it's two things:
Firstly, as you suggest, is a lack of economies of scale, startup expenses, high R&D costs, etc. Existing car companies can do a lot of the EV manufacturing with known techniques, existing machinery, etc.
But secondly, at least Telsa has an absurdly high profit margin compared with traditional high-volume models. They're taking the luxury route of a smaller sales volume, higher per-unit costs, and making a good profit off each vehicle.
In short .. because they can.
A lot of companies are also adopting the Tesla model of starting at the high end and working down. It's a lot easier to build in margin on higher end products. If companies start at the low end with a cheap EV, they may not be able to meet demand. The start ups also don't have the manufacturing chops of the older car makers, which is why you see them making really expensive cars... they know they can't scale yet. Ford on the other hand has priced the Mustang EV and Lighting pretty reasonably. I see the Mach-E all over the place and I'm sure I'll see the Lightning everywhere soon.
In Europe and America most of the EVs on the market (maybe due to regulation or to car makers greed) have huge batteries with DC charging. Said batteries are extremely expensive compared to ICEV fuel tanks, that do the same job - store the energy to move the car.
EV batteries take up a lot of space and that likely complicates assembly. My plugin hybrid has about 20 miles of EV only range, and loses 5 cubic feet of storage for the extra battery vs the hybrid model; 5 cubic feet is about 37.4 gallons.
I suspect battery costs are higher than you think.
Billions have been invested in engine and transmission design and manufacturing optimization. Engines are surprisingly cheap to make.
Emissions and efficiency requirements have made them more complex.
I wonder if the last hurrah will be an engine with fully electrically controlled valves, like Koenigsegg already has. Getting rid of all belts and chains could be great for reliability. All pumps would be electric.
It's impossible to ignore the supply chain problems as there were not EVs available prior to 2021/2022 from most manufacturers.