[Someone]

As others pointed out, you had to, because of lack of registers.

Also, the article explicitly mentions it does more loads and stores than necessary (”In real life, values from one expression will remain in registers for the next, and so won't need to be reloaded; the examples are all deliberately choosing the worst possible case.”)

Finally, writing to memory wasn’t as bad in those days as it is today (or rather: using registers wasn’t as fast as it is today). Writing to a fixed address, for example, only took twice as long as a register-to-register move (4 cycles vs 2 cycles on a 6502, if I googled that correctly)

[Gibbon1]

You are are correct, direct memory access on those machines were really cheap compared with today and even earlier. Modern day memory bus speeds haven't kept of with processor speeds (this is what sank RISC machines). Old core memory is slow. Drum memory is hella slow.

The other thing is there is a trade off between number of registers and instruction size. With 8 bit machines you see that for instance where only certain addressing modes can be used with certain registers. You don't have enough instruction space to encode for every addressing mode for all the registers you have.