Why SPF over PTR?

[Ludwig77]

I must not be understanding SPF correctly because I don't understand
why it is better for preventing sender domain forgeries than doing a
simple PTR lookup.

Wouldn't a simple comparison of the sender's IP address to their
domain name via a PTR record lookup be sufficient to check for
forgeries? I realize that not everyone has PTR records, but more have
PTR records than SPF records.

What am I missing?