<p>It seems that a Bowden extruder is the most used in all cheap 3D printers by far compared to Direct Drive that is very rare under 500 USD machines. But I haven't understood the reason, since in terms of hardware a direct drive doesn't seem to have any impact on price more than Bowden (correct me if I'm wrong).</p>
<p>Why?</p>

As I understand it, there's really no good reason for this except "momentum". At some point in the not too distant past, a Bowden extruder was seen as an "upgrade" over direct drive, which required a bulky toolhead that was seen as limiting speeds.
(This perception was at best accurate only for delta and CoreXY machines at the time even, I think. As it turned out, Bowden doesn't let you print faster, at least not at any quality, because the nonlinear/hysteresis effects of the Bowden tube on the actual amount of material extruded can't fully be compensated with linear advance/pressure advance once you reach moderately high speeds. You can overcome this with the
Nitram Bowden
but good luck finding a cheap 3D printer manufacturer willing to put in that kind of custom part!)
Anyway, all the cheap printer manufacturers jumped on Bowden as a feature, and they're slow to develop any new designs rather than just making incremental improvements and production cost optimizations to existing ones.
Since then, direct drive designs have improved greatly, and the mass of the good ones has gotten so low that it's hardly a consideration anymore except on the most extreme agility-seeking printers (designs attempting 50k-300k acceleration). Everything should be direct drive, especially since it makes things so much easier for beginners (no difficult-to-load tube, broken filament in tube, loose fittings messing up retraction, etc.)
Teaching Tech has a video, oddly named
Why direct drive is not automatically better than bowden tube
, where he basically concludes that it is actually better, and goes over some of the history I've touched on.