100% of all PC cooling fans from 90s to today, from 40mm to 200mm, from budget to most expensive high performance models, they all use thin blades.
Why?
Airplanes wings and propellers use more complex thick "airfoil" shapes. Wouldnt a thick airfoil shaped blades offer better airflow/static pressure?
Airplanes need their wings to be structural. The entire weight of the fuselage is supported by the wings. For this they need to have some thickness. They also contain most of the fuel.
Airplane wings also need to be stable in multiple conditions from near stall during landing to high-speed and high-altitude cruise. And you will want to ensure that if there is a failure in the flaps that you can still fly slow enough that you can land.
A fan on the other hand only needs the blades to support themselves and only have a narrow range of conditions they operate in. And loss of thrust/lift on one of the blades is not an immediate cause for panic.
Optimizing a fan for airflow means a few smaller blades while optimizing a fan for airpressure means a lot of overlapping blades to prevent backflow.
Not to mention it's less material and weight. Which keeps the manufacturing costs down.
It's because of scale and Reynolds Number. For air moving over a large aircraft wing, where Reynolds Number is large, the ideal shape of the wing to generate lift is as shown for the more recent wing cross-sections in the question's image, and looks rather fat. However, at low Reynolds Numbers - such as birds - or even more noticeably, insects - wings tend to be thin. Bird wings are like the Wright or Bleriot wings, while insect wings are usually thin, flat, slightly ridged flexible plates.
Consider this article.
So, for a fan blade which would have a much lower Reynolds Number than an aircraft wing, the blades are closer to those of a bird or even an insect.
There is an additional advantage in that requiring only a thin wing in order to generate the required lift and consequent airflow means that there can be cost savings on material for the fan blades. They don't need to be made thick, from expensive material when they can just as easily be thin.
Also, as shown in the question's second image, Noctua is known for producing especially quiet, long-lasting fans. Unfortunately the fan in the image is facing the wrong way, but their fan blades have ridges that reduce turbulence and noise that cheaper fans don't have. These ridges can be seen on the edges of the upper-left fan blades.
The pc fan is thin as it only needs to move air from one side to the other. Simple as using a thin plywood sheet moved through the air to fan a barbie.
A wing for an aeroplane needs to generate lift which happens when there is a pressure difference generated between the two sides of the blade based on the different shapes of the two curves.
It's not significant enough for makers to care.
Differences in wing shape generally only translate to some small percentage performance difference. https://www.researchgate.net/publication/374514970_Influence_of_Wing_Shape_on_Airfoil_Performance_a_Comparative_Study
For aircraft, with the scale of money involved, even these percentages will still translate into large cost differences, ex. with respect to fuel efficiency.
Compare to PC fans, where even the "high end" models are maybe only a few hundred USD a pop, and efficiency of the fan itself is unlikely to directly translate into much of a difference in sales etc. - the cost of researching and engineering this aspect of PC fans likely far outweighs the potential benefits.
As an aside, PC fans seem much more comparable to jet turbines than wings - and resemble them much more closely. This is in all probability the "good enough" design for PC fan blades.
