Do I Know? Nope -- I'm afraid not.
I do have notions of why, though:

1) Even the most horrific crash won't exact that much stress on these (rather large) diameter bolts.

2) They're less expensive. (If you don't need the extra strength, why pay the extra cost?)

3) Inherent tamper-revealing softness. (My own pet notion) Litigation could come to pass on whether the accident scene held originally-installed seats+bolts, or if they'd been subject to previous seat-reinstallation. Factory liability vs. innocence.

BTW, I've gone to 'black-oxide' bolts, myself.

 

 

As mentioned above. The bolts that porsche used are strong enough to meet safety standards but also cost effective. I worked a company that made porsche seat mounts and harness equipment and saw many seat bolts for that 993 and 964 that were nearly stripped out as shown above. The hex portion is from what i remember shallower than normal.

 

There is some guy on the 928 side, can't remember name, also on eBay selling hardened hexheads for Porsche seats at $10 for a 2 seat package. Don't know if that is a good price or not.