why can I not buy an MacBook with 24 instalments?

This is primarily a user-to-user support forum; other Apple users like you. We don't have access to unpublished information from Apple. And I've never seen anything published about "why."

But the following does verify that Apple Card Monthly Installments for a MacBook are 12 installments and that there is not a l24 installment option: Products you can buy from Apple with Apple Card Monthly Installments - Apple Support

We as users have no insight into Apple's policy; we cannot intervene with Apple on your behalf. You'll have to ask Apple: Official Apple Support

The OP may wish to consider the 48 payment option as well.

"installments"

You can. Make twice as many payments of half the amount over the same period of time.

LOL. Had to think about the math there for a moment. Indeed, 24 payments in 12 months ... got it.