To be fair, grandparents are not the primary users of MacBook Airs.
Seriously, the MacBook Air is the highest-selling model of Mac. That translates into both a significant number of new sales and a significant number of replacement sales (of previous generation models) every year. Both justify the MacBook Air always being on the latest generation of M series SoC. The MacBook Pro also has a very strong annual new/replacement sales cycle and justifies generational updates.
The Mac Pro is on the opposite end. It makes up a single-digit percentage of "professional" Mac sales per Apple's comments in 2017. And those folks who drop five-figures on a Mac Pro tend to not replace it every year, even if the revenue streams they generate from the machine would allow for such. And this is not a unique situation for the Mac Pro, but applies to all desktop Mac families (Mac Studio, Mac mini and iMac).
Also, desktop form factor models make up the significant minority of Mac sales (less than 20%) so while there are new and replacement annual sales, the volume of them pales compared to the MacBook Air and MacBook Pro lines. Therefore, not being on the latest and greatest generation is not as detrimental to those models as it is for the portables.