they don't? There's no company, or rather - a lot of them, Linux kernel moves forward like 80% by corporate contributors. For some of them it's critical part of their infrastructure, some of them need to get their device drivers mainlined, for some of them it's gpl magic at work. Linux desktop experience, however, leaves a lot to be desired.

Companies aren't interested to contribute to a browser when they can just reskin chromium or build on blink directly and community cannot match the pace.