The PS5 uses a centrifugal blower rather than an axial fan. Blowers can push air effectively through restrictive heatsinks and vents, but at the cost of greater turbulence and therefore noise. The Xbox Series X/S consoles use axial fans that move a lot of air at low RPMs; this requires large intake and exhaust vents and an optimised internal layout, because the flow rate of an axial fan falls rapidly as static pressure increases.

The PS5 is fairly loud, but the noise is quite directional - it's much louder from the rear than the front. By keeping all of the vents on the front and rear of the console, it can work effectively inside a TV cabinet or media center that would suffocate an Xbox. It's not the set of tradeoffs I would have preferred, but it isn't a thoughtless design.

One fan might not be enough for the amount of heat generated. The airflow engineering could be very poor, requiring high velocity and not just high cfm to overcome. It could be positioned poorly or attached to a skimpy heatsink to save on copper (or even worse, to save on another metal). The hottest components may have very small surface area so they don’t diffuse heat efficiently. Lots of reasons, usually having to do with thermal engineering not being a priority until late in the game.