Not a stupid question.

Between the training required for a solo parachute jump, and the cost (and more importantly) weight of the equipment, plus the relative safety of commercial flights, it's simply not justified.

In more than a few cases we've seen airliners make emergency landings that are gnarly, but the majority survive. In more cases than I can count, there's checks and balances that ground flights because of safety concerns either at the departure point or at the destination (icing, high winds, etc), or due to mechanical concerns.

It's rare that a fully inspected and functional aeroplane will fall out of the sky, and we do everything in our power to ensure that all planes that leave the ground are fully inspected and functional. Short of a freak occurrence, like a fast forming weather phenomenon, there's so many checks and balances that airliner crashes are exceedingly rare.

So not only is a crash rare, there's no guarantee that a crash will be fatal, usually the pilot can at least get the plane on the ground without killing everyone aboard, and the fact that it's a massive amount of extra weight that requires training that the average person doesn't have, there's little point and nearly nothing to gain from doing something like that, while it would have significant downsides on flight efficiency and increase the costs of fuel per flight due to the extra weight.

Then there's the consideration of, even if they were able to successfully parachute to the ground, what then? It's pretty much a guarantee that nobody has a radio, and that you're far enough away from civilization that your cellphone doesn't work, so now you have hundreds of people spread out over potentially thousands of miles of terrain/water/whatever that you now need weeks to search and rescue everyone. Taking weeks on search and rescue, pretty much guarantees that you'll find people who landed safely, then died from exposure to the environment.

On the flip side, if everyone is in the plane when it crashes then all you need to do is find the plane; everyone will be in that general area, whether alive or dead.

There's just too many downsides to having parachutes on board to make it feasible.

Throwing untrained people out of a commercial airliner at high speed in the middle of a emergency is a good way to ensure no one survives. The equipment would add a significant amount of space, fuel and maintenance burden too, and require major compromises to the aircraft design itself. All to resolve a problem that effectively never happens.

An explosive release canopy for an ejection seat system on an airliner would just release the entire top half of the plane, and don't forget that fighter pilots are both wearing flightsuits and get specific training for the event.

Even beyond the material and engineering costs it's a difficult ask, probably better to just focus on reliability in the first place.

I remember seeing an article back in the 90s or maybe even 80s that was exploring the possibility of the entire passenger compartment separating from the wings and rest of the fuselage and parachuting down in the event of a major failure. The thing is, it would be ridiculously expensive to implement, and there are very few situations where such a system would be any better than keeping the plane in one piece.

Rigging a commercial airline with that many ejection seats would add significant weight to the plane. You'd probably triple the cost of commercial airflight if you did this just from reduced seating capacity, and even assuming that it could be implemented without that overhead I still don't think this would actually help much.

Imagine you're on Ethiopian Airlines Flight 302 in a 737 Max nosediving towards the ground and the roof just opens up and launches you and all 148 of your fellow passengers out of the plane at 400MPH. Somehow I imagine that you just end up scattering the mangled corpses over a wider area.

Parachuting isn't as easy as pulling a wire and gently floating to the ground. Those who parachute professionally take hundreds of hours of training. If you're brand new, you're required to strap yourself to a professional.

Some valid answers are already given by other commentators. Just want to highlight that commercial airlines are operating barely cost positive. Every extra bit of cost added has to be at least covered by some other stream of revenue. How much more money can a seat in these crammed airliners make to cover the cost of R&Ding and maintaining additional safety measures?