There's a few reasons that probably interact with each other a bit.
#1: Math.
Rolled stats (4d6 drop the lowest) does yield a higher average than point buy.
#2: Psychology.
People tend to post things that reflect well on themselves and avoid posting things that reflect poorly. Thus, you're far more likely to see what amounts to "hey, look how lucky I am" than you are to see what amounts to "lol, look at this thing that sucks." That's not to say that self-deprecation isn't a thing, but it's usually a means of ingratiating oneself to an audience than it is a general communication strategy.
#3: DM Policy.
Most of the DMs who I've played with as a player, and most of the DMs who I've spoken to as a fellow DM, will allow a reroll if the stats end up ranging from profoundly bad to average in every regard.
#4: Survival & Attachment.
Generally speaking, genuine attachment to a character takes time. In order for that period of time to elapse, the character must survive that long. Depending on what stats you rolled poorly on, a character may not have much survivability. This is more of an issue where stats are forced to be rolled in order, but it can occur where a player makes poor assignment choices.
For #3, a total re-roll for a poor set of rolls was not just the judgement call of a poor DM, it was part of the rolling rules for some variations of D&D. It could be described as 'fudging' if you want, but re-rolling poor sets of rolls is, like I've said before, a
feature of rolling, not a bug!
Rolling, with all the ways to mitigate poor rolls, is closer to the ideal of partly controlled/partly random character creation, and neither
un-mitigated rolling nor point-buy do the job.
There's also been some debate about whether to roll first then decide what kind of PC to create, or to decide first then hope you roll stats that support that concept (or use point-buy). Point-buy is obviously geared toward the latter, but D&D has, from its infancy, expected the former.
It has to be said that 'roll first, then decide' reflects real life better. Most people are what they are. They can't just decide to be more intelligent! Point-buy does allow this; usually by voluntarily making themselves weaker(!), but in real life you are what you are and then have to decide what to do with your life, ideally playing to your strengths.
Sure, you might have pushy parents who expect you to work down the mines like my father and his father before him, even though you have the soul of a poet.
"How old are your children?"
"The doctor's three and the lawyer's five."
So, a player staring at a set of rolls he just made (the second set actually; the first set was so poor that the DM let him roll a new set, just like it says to in the PHB of the edition you're playing) has to decide what class to be given those rolls. This reflects real life quite well.
Actually, my main group designs the character before the stats are known. So, we must use point-buy, right? How else can we be certain to get appropriate stats for the concept?
No, we never use point-buy. We just choose our stats; no rolling, no point-buy.
That's how to narratively create a PC.
