Android is the Wild West, with all the good and bad that follows. Manufacturers are often not implementing things properly. Problem is that these devices are all over Amazon and people buy them and expect them to work. We try to steer people to good quality devices such as the Nvidia Shield but you can only do so much.
Keep in mind that GeckoView may not be used to build a browser. Rather it's often an engine used to drive an application built with web technologies. So a developers requirements for updates are different than an interactive browser of arbitrary web content.
The story is the same on the desktop with Electron. When you deploy an electron app the browser underneath it is locked in conjunction with the version of Electron.
It's still up to the app developer to be responsible and follow issues and update as required. But that's a lot less often than when you have a browser of arbitrary web content.