Isn’t the foam lining replaceable?

A helmet is like an Ogre (or an onion, but never parfait).

I did a quick search for a diagram to illustrate this, and this one will do:

On the outside, there will be a hard layer, normally made out of Fibreglass, Polycarbonate or Carbon Fiber depending on your budget and tolerance for weight. Beyond simply protecting the next layer, it's primary purpose is to spread any impact trauma across the next layer.
Under that will be a thick layer of expanded polystyrene foam - this is what saves your head in the event of unplanned rapid deceleration.
Under than will be some comfort padding, normally attached to removable liners, which helps to keep the helmet comfortably in place on your head. This is the layer can be removed for washing/replacement.

The one we are really worried about degrading is the polystyrene one, which isn't readily replaceable.

The outer shell is normally sculpted somewhat to assist with aerodynamics, which decrease wind buffeting and noise for the rider.
There's also vents and air channels, visor ratcheting mechanisms and on some helmets provisions for bluetooth communications systems, but that's a whole other story.

Your helmet is arguably the most important piece of kit you have, and the groupthink is to pay as much as you can afford about every 5 years (whether you ride every day or bi-annually).