With hookup material there are less criteria and requirements pretty much something along the lines of:

  1. The guy being attractive,
  2. Having a larger dick (if you are a woman into that)
  3. Personally I'm more into younger guys so younger age
  4. Has good rizz. That's was pretty much my criteria for a hookup with bonus points if they are kinky.

For relationships there is far more criteria: 1. Has good morals 2. Is a good person 3. Is polite 4. Is attractive 5. Has a career 6. Has a good education 7. Has a lot if similar interests 8. Is affectionate 9. Gives me a lot of attention 10. Lasts a long time during sex 11. Has a bigger dick

So as you can see there are a lot more criteria for a relationship so being seen as only good for sex is kinda an insult really.

Not to mention a lot of women regret hookups and don't find them satisfying and don't orgasm. Only 40% of women had an orgasm during hookups. Women were twice as likely to reach orgasm in serious relationships as in hookups. Both men and women sometimes experience hookup regret, but Garcia found women are twice as likely to have a negative reaction later on.

https://time.com/archive/7135161/no-satisfaction-woman-are-less-likely-to-orgasm-during-casual-sex/

Personally I found hookups dissatisfying and didn't orgasm. So as a male how would you feel if you did have a hookup and the woman was dissatisfied and didn't orgasm? Honestly sounds bad to me. Being partner material is much better and the woman is more likely to be satisfied and orgasm.