Because the brand new MacBook is still slower than my 3-year-old i7 Air. I'm a developer, and I need speed still, too. If Apple could fit a competent i5 or i7 in the MacBook form factor, I'd be all in.
I switched to the 2nd gen m5 Macbook from the '13 rMBP 13", and would argue the m5 is surprisingly more than capable.
Granted it's not as fast on paper or benchmarks, but it's so capable I am having a hard time trying to justify my next purchase (not enough screen real estate in rarer circumstances: Chrome DevTools alongside the browser window is impossible, OmniGraffle and other apps can feel cramped at times).
The machine did feel a little slower than the i5 when I first started using it but nothing obnoxious.
Context: As a devops guy I regularly build large RPMs, run grunt against a huge codebase, compile packages from source, and all the while with Docker running OpenShift and a bunch of LEMP apps in the background.