A big part of the problem is that people can't leave their bikes outside due to a lack of secure space. Bikes are often expensive and if left in a shed, they will disappear. Hence they get taken inside.
I think there is a lot of fuss out of all proportion to the risk, just to get clicks or sell papers. I don't think it's fair to tar all home builders with the same brush. The percentage of people who build their own batteries out of all e-bike users must be incredibly small and those that do generally give up, or learn a lot in the process and end up with something equal to or better than commercial battery packs. A very very tiny number end up bodging something together and use it recklessly.
On this site, when discussing battery builds/repair I have always seen people with suitable knowledge stepping in to point out the dangers associated with what someone might be planning to do. It is better to have people coming to these sites and be advised not to do stupid things, or things they do not have the competence to achieve than have them mess about on their own.
Let's not forget that our homes are full of Li-ion powered devices, from laptops, to shavers to disposable vapes. The standard of construction varies from one manufacturer to another and any one of them could cause a fire. Just because a system is commercially made is no guarantee it is safe. I'm not just talking about Li-ion based systems...Grenfell was started by a faulty fridge. Other domestic fires have been caused by washing machines, dishwashers etc etc.
For Li ion batteries, chargers are a big issue, too many of them output a voltage that is too high, causing stress on batteries and BMS devices.
Then there is the stupidity factor. I could easily plug a 48V charger into my 36V battery, the connector is the same. I know not to do it, but others don't. What if your fuse goes on your commercial battery pack and you stick in a replacement that is a higher rating?
I have bought several Yose power packs. A reasonably big brand who try and do things right, but when I have taken them apart, there are a lot of things I don't like from a safety point of view. Yose power are not alone in this. This has led me to dismantling most of my commercial packs, replacing the BMS and wiring, connectors, switches, fuses and then re-packaging.
Extra regulation and being forced to buy only from commercial suppliers will not stop battery fires.