No genuine question is ever dumb. If someone legitimately doesn't understand something and is trying to learn, then we can help them learn. We don't chastise people because they weren't exposed to information we've already learned.

I know some of this, but not the whole answer.

One really important thing for helmets, carseats, etc is whether the material is multi-use or one-use.

One-use materials are generally far lighter for the same level of protection (protection itself a multi-faceted concept but I’m sticking with simplicity here).

Hockey helmets are generally lined with vinyl nitrile or polypropylene foam. Vinyl nitrile is the same stuff thats in HVAC gaskets, yoga mats, and all sorts of seals. It returns to its previous shape when you are done squishing it.

The inner foam of a motorcycle helmet is typically expanded polystyrene. It is designed to collapse and absorb force in an impact. Polystyrene foam is what many foam cups are made of (though obviously very different in structure in a helmet than a cheap cup). That foam has a bit of bounce and flex, but it is designed to permanently crush/collapse when it gets hit.

Both helmets have an outer shell that is designed to spread an impact over a large area.

But overall: multi-use vs one-use. Skateboard and hockey and snowboard helmets are multi-use (and weigh more for the same level of protection). You replace them when you take a massive hit that cracks the shell or sometimes after you use them a ton over time. Bike and motorcycle helmets and car-seats have to be replaced once they do their job - that foam does not work twice, but it’s much lighter for the same level of protection and used in applications where you don’t ever plan to actually need it.

Honestly a fair question since all helmets are designed for hits to the head.

They all protect for a specific type of hit to the head. Simplifying your example to a bicycle helmet vs. hockey helmet: A bike helmet is likely to be hit directly from the front, it is lightweight for ease of riding, and it will sacrifice itself to absorb as much impact as possible, making it single use only. Hockey helmets are designed for multiple hits, and protection from a projectile. H hockey puck might punch a hole through a bike helmet.

Motorcycle helmets are relatively big. Any worth buying will cover your entire head, including your face and eyes. The good ones will even extend down and squeeze your neck to create a good seal. This is good when riding a motorcycle because it makes for a quieter helmet. But for sports this means that your head is trapped in this almost air-tight hot box. Of course this affects motorcycle racers as well, but they've got the benefit of > 100 km/h winds entering the air channels and cooling them. Hockey players don't.

Motorcycle helmets are also considered one-and-done deals. (although this is heavily debated within the community) They've got a great, thick layer of impact absorbing material, but once that material has been hit, it loses its effectiveness.

TLDR: motorcycle helmets are bulky, heavy, and are designed to cover the user in a way that's detrimental to playing hockey. Playing a physical sport in one would be horrible. And their materials may not be suited for the use.