Because money matters. And while officially Linux is not owned by Google, de facto Google ownes it and dictates what license it going to use.
Linux is used to run Android. Milliards of phones are sold with Android/Linux and Linux part is affected by GPL.
GPLv3 would be great for that customers and very costly for Google because instead of buying new phone with increased version of Linux kernel, or upgradable "official" phone which costs 10x more, people would be able to demand that their software does not comply with license, and hence is illegal.
It is also illegal to deny consumer rights to control, meaning locked programs inside phones etc. That is what GPLv3 stands for in first place. Denying of basic consumer rights is "legal weaponry" used by Google to kick out small players out of market share, and all of that while using free software in which people invested their time and passion during 1990s.