Tesla builds EVs, that's the only thing they sort of can do (of course they can't actually build them at profit, but that's a problem for Tesla's shareholders). Others will build EVs when they can be profitably sold, because it isn't their only business.
Tesla's margins are the highest in the industry for their vehicles. Legacy automakers don't want to innovate and cannablize their own business, which is why they cannot succeed.
If you want to sell like Tesla does, and get the level of subsidies they do, sell vehicles that people want and will pay for. There's a reason people would rather wait years for a Model 3 then buy a Bolt, i3, or other half-attempt at an EV by a legacy automaker.
People want to buy a Tesla for the same reason many want to buy an Apple or Bose product -- great marketing, not any kind of intrinsic value. And besides, "legacy" automakers (you know, those guys who actually were first to make an electric vehicle, hybrid, fuel cell, hydrogen etc., ones who have driver assistance systems that do not try to actively kill their users, etc. etc.)already have cars people want to buy. Unless you're talking about some edge cases (doesn't Norway tax regular cars so much that Tesla is actually a good value?) making EVs for them isn't currently a very profitable strategy. Not that it were profitable for Tesla either, of course.
It would also be interesting to learn what is it that Tesla actually innovated, apart from marketing (and even there, Apple or Bose has been selling flash over substance for years and years before Tesla)? Batteries? No, they are completely off the shelf. Do they have some magical mystery motors? No... Some exotic body construction maybe? Nope, aluminum cars have been around for quite a while, too. Some incredible self-driving software? No, better not even go there.
It will be interesting to see what happens in a couple of years when VWs electric factory comes online.
