What sells me on it is I get to take a spare Gen4 m.2 ssd out of my gaming PC I wasn't fully utilizing instead of paying for 1TB of storage.
Being able to drive the price down by re-using parts I already have is a pretty big selling point, IMO.
Also, I think Apple is benefitting from scale, since they're able to maintain the (usually too high) storage and memory prices they've had for years. At this moment in time, framework have the misfortune of being forced to pass inflated wholesale prices onto the consumer.
Make this comparison one calendar year ago and the F13 Pro could very easily beat the MBP on price spec-for-spec.