No Stupid Questions

TL;DR: Because Saving Private Ryan is a movie, and meanwhile reality is reality.

Whether or not a helmet can stop a bullet (and manage protect its wearer in the process) depends an awful lot on how much energy it has to dissipate, i.e. how fast the bullet was traveling and how much it weighs.

Rifle bullets travel very fast. This has not changed appreciably between WWII and today, although contrary to expectation it was more common to have front line soldiers issued with full power battle rifles back in WWII which were actually more powerful than the intermediate cartridge rifles most often issued to them today. Military rifles nowadays actually commonly fire a much lighter bullet than in the past. (Yes, there are exceptions. That's not really the point.)

There is no such thing as any kind of metal helmet that can protect the wearer against a rifle bullet that is a square hit and within the rifle's optimally effective range. You can play with ceramics like are used in plate carriers that protect the torso, or weird high tech aramid fibers, etc. but the long and short of it is that such a thing would be too bulky and heavy to feasibly wear on your head. A bog standard 7.62x39 round, i.e. that fired from an AK pattern rifle commonly found all over the world, delivers around 1000 ft-lb of energy at impact within 100 yards. Even if you could magically stop it somehow it would ring your bell like you wouldn't believe. We're talking unconsciousness, fractured skull, brain damage.

And to put it into perspective, the 7.92x57 round fired by the types of rifles likely to be issued to the Germans during WWII was even more powerful than this, developing around 2,900 ft-lb at the muzzle (I can't find a figure for at 100 yards offhand, but just subtract a couple of percent). Yes, that's around three times more powerful. You are therefore much less likely in reality to be happy about being shot in the dome with a WWII battle rifle with a primitive WWII helmet versus a modern helmet and a modern intermediate power cartridge.

A steel helmet stands a greater chance of deflecting a pistol round which is slower and carries considerably less energy. 9x19 round at 100 yards is packing more like 250 ft-lb of energy, a quarter as much as the 7.62, and is also shaped with a much wider cross section and a less pointy nose so it's less likely to penetrate hard objects.

Any garden variety lid would be much more useful at deflecting a shot that was a glancing blow, or that was fired from a very long way away, and/or has ricocheted off of something and thus lost much of its energy. Not to mention fragments of whatever it hit -- bits of brick or doorframe or glass or whatever it was the enemy's bullet hit near you that was not you. And shrapnel, and gumpf raining down on your head from nearby explosions, etc. Helmets are designed to maximize their effectiveness based on what we understand and can build (and, yes, what the lowest bidder can manufacture) but are not and never have been expected to shrug off a straight-on headshot from an enemy's rifle because this is a fool's errand.