My thoughts on this subject as a very experienced no-recip top who has of necessity given the topic a lot of thought:
1. Many of the guys offering blowjobs really want to get fucked, but won't say so online, because it's less popular, considered more risky, more sluttish, gay, HIV, etc. But be sure that if you actually do hook up with these randos, most of the time they will want butt sex. You will see the occasional top post that he is only looking for oral, no anal. He says this because he must, to keep the secret bottoms away. Even so, when I post I'm looking for head, I still get guys wanting me to fuck them. I sometimes remind guys that I'm looking for head and not ass, and they disappear. I disagree some with the OP - most of the guys I encounter want to get fucked.
2. There's an element of addiction to this. Guys want more and more cock. The hunt itself is somewhat self-reinforcing, as people send you messages, pics, etc. Don't discount the number of antisocial crays that are online now as well, entertaining themselves by pretending to look for cock to suck.
3. You can't offer many blowjobs if you want reciprocation. You won't be horny after you cum and won't want to blow anybody. No recip keeps your stamina up - potentially you can blow more guys.
4. The guys who want no recip blowjobs are a type sought after by bottoms - they are typically more masculine, more horny (testosterone, basically), sometimes married or closeted which appeals to some. These so-called trade tops (now called 'total tops') are also pretty rare in the hookup biz. Bottoms tell me all the time they rarely meet a total top.
5. Guys have sex performance issues because they're obese and on SSRIs. They can't get it up so it's easier to offer no-recip head and avoid embarrassment.
What really annoys me are all the gloryholes. In people's homes or in hotel rooms. Often poorly made with rough edges, odd heights and angles. No thanks.