Because a year ago, the average was about half of what it is today and therefore we will be out of space in a other year, never mind spikes, never mind unforeseen factors, never mind the time it takes to get consensus and do a rollout.
Okay. The average block size is still less than half of the limit. So why couldn't Gavin and Mike continue to work within the core community again?
Because of precisely what I said in the OP. Greg wants us to literally run out space before doing anything. He and the other core devs don't want to act.
And Gavin doesn't agree. Nor does Mike, Jeff, a handful of major companies, and an increasing majority of miners.