btc has put in some fixes for the problems though, everyone is now waiting for them to be adopted. Coinbase could cut their fees if they implemented Segwit. There are more improvements coming like schnorr signatures, and then obviously side chains. The philosophy has been to try the solutions they think will work before they increase block sizes. There are good reasons why some forks are called scams, especially if they are ran not as an open source community but as a company with a CEO.

The nature of bitcoin makes it one that the developers should be very cautiously improving and very skeptical of proposals. The linux kernel is the same way. The high stakes requires everyone to scrutinize changes, especially corporate backed ones. There are real concerns about raising block sizes as one of the key ideas of bitcoin is preventing scenarios that can lead to centeralization. "Core" is a large open source community, not a single leader who rejected raising block sizes for political reasons.

Bitcoin Core developers still didn't implement a SegWit wallet. I guess Coinbase can't be sure that their SegWit implementation is going to be used correctly.