They COULD contribute their efforts to improving EXISTING languages (like MSFT finally did to their credit with TypeScript, building on non-Proprietary JS, which is great) but there is a seemingly irresistible allure to the thought of owning the design and molding a language to perfectly fit your own company's desires. You never have to deal with standards, when you're just making it up as you go along. PLUS the massive benefit of the anti-competitive foot-hold (monopolistic foot-hold) that having its own language gives to a company, is just too tempting. They cannot resist.
However most developers are sick of the lanuage-Thrash. 99% of it is not innovative in the slightest, but just somebody reinventing and resolving problems that have already been invented and solved countless times before. I really really hope dart dies (and GO can also go). In my opinion there are three languages that need to still exist: Java, JavaScript/TypesScript, and C++. Those fill EVERY need, and if the innovators would ADD TO THOSE rather than starting from scratch and pushing the reset button the software world would be a MUCH better place, and so would overall software quality.