But they did exactly that with the M1, on the MacBook Pro 13".
Apple likely just swapped the M1 into the Intel chassis for flexibility and time to market reasons.
When it came time for the M1 iMac, for example, they waited some six months and released a new form factor rather than slap it in the 21.5" iMac case and include it with the MBA, Mm and MBP 13.3.
And with the M1 Pro and M1 Max launch, they introduced them with new form factor MacBook Pros.