Fifth gear can see much higher tooth loads.
For a given vehicle load the torque at the gearbox main shaft will be the same.
Now this torque at the main shaft is the same for all gears, but because fifth gear (on the main shaft) has the smallest pitch circle diameter, the tooth load will be higher (think of lever principle using the pitch circle radius).
But fifth gear on the gearbox main shaft besides having highest tooth load also has a smallest number of teeth than its mating gear.
The tooth profile of gears with small tooth number count is such that the thickness of the tooth near it's root is smaller compared to a gear with more teeth (assuming same module teeth) - this is the nature of involute tooth profiles. The tooth load creates high stresses at the root of the tooth, so the gear with a smaller number of teeth will not be as strong (everything else being equal).
Now the other issue is with wear, which is a primarily a function of Hertzian contact stresses. It turns out that the tooth profile for a gear with smaller number of teeth has a smaller radius at the contact points on it's flank - this is the nature of involute tooth profiles. This is the main reason why the Hertzian contact stresses are higher. The other main factor in Hertzian stress is surface hardness, but this is a material property nothing to do with tooth profile.
In all gear design, where different diameter gears are in mesh, the designer has to evaluate the strength and wear of both the smaller and larger gears (4 calculation for each gear pair).
Usually the pinion (small gear) is made from higher strength material than the wheel (not waste $$ on material not needed for wheel strength/wear).
The other things that designers can do is apply positive addendum modification to the pinion - this results in increased thickness at the root of the teeth. But there is a limit to how much addendum modification can be used because the tip of the teeth can get too thin which can be a problem for case hardening depth (case hardening materials produce the strongest gears).
The other main factor that the designer can control is the tooth width. Width is increased to increase tooth strength/wear capacity. But there is limit in how much the width can be increased, over which the returns diminish because the loads are distributed uniformly over the tooth width (mainly due to deflection).
IMHO, the gearbox designers have taken the view that drivers will/should use lower gears for pulling heavy loads and have saved $$ in the design of fifth gear.
Edited 3rd sentence to fix error in logic (brain fade when fist written) and small change in 4th sentence for clarification.
