The A18 is already faster than the M1 in single core processing and about the same in multi core. The M1 was built on an old chip process that Apple and TSMC would like to retire. They are likely producing few of them any more.
Apple isn't selling the M1 Air. They do sell some through 3rd party sales chains but that is a lower margin channel. This new MacBook is a way for Apple to reclaim those sales and get a better margin when they do.
Making it a dock for your phone would be a completely different product for which there is no current hardware or software support. They would have to put a lot of effort into that and it would confuse their product lines. The hardware would also probably cost about the same as the chip is not the most expensive part of a computer.