That's certainly the line that can't be compromised, ultimately you fly because you wanna get somewhere. But finding and choosing favorite airline(s) has many perks that aren't easily quantifiable, enough that many frequent flyers choose to focus their patronage for those benefits.

I'm personally a Delta fanboi because Northwest was my childhood airline on family vacations. My Delta SkyMiles account goes all the way back to 1998 when my dad opened it for me as a kid, back then it was Northwest WorldPerks.

Small tangent story: Delta SkyMiles was instrumental in my getting an AMEX credit card as my second ever card with merely 6 months credit history as a literal newcomer nobody to the credit world. It's been my favorite card to carry ever since.