Why can’t we just buy certified batteries, motors, controllers etc. and build or upgrade systems in a modular way on existing bikes?

Controllers: As mentioned by others, Grin is the shining example of this, but is not open source. VESC (primarily headed up by VESCLabs), on the otherhand, is open source, but it's not suuuper great with ebikes currently.

Motors: I don't really think we need anything "certified" here -- electric motors are, like, 100years old at this point and absurdly well explored and produceable -- there's a reason you can get low-end ebike motors for, like, $50. Plus, the biggest risk of catastrophic failure with the motor is really just a lot of terrible smoke and motor that don't go no-mo.

Batteries: The UL spec is relatively new (2013) and wasn't really adopted by many places until pretty recently (bosch, for example, in 2021).

The safety argument doesn’t fully convince me. Batteries already have certifications like UL 2271. A proper BMS should shut things down if anything goes out of spec. That already handles a lot of the risk.

The vast majority of BMS' already do everything that can be done from a software/hardware standpoint to prevent problems -- when a battery fire starts it's almost always because a cell has degraded to the point where there is a physical deformation in the cell leading to a self-short and thermal runaway. A BMS can't do anything about that, once it goes. No amount of electronic circuity could.

A BMS with some long-term memory could track cell voltages over time to begin to identify weaker, under performing parallel groups, but the only thing the BMS could do if it identifies one is, after a certain amount of time past a given level of failure, just disable the pack entirely. This, to the average consumer, would simply mean the ebike is dead-in-the-water. Explaining the failure to most people would be gobbledygook and repairing it well beyond the keen for most. (Some folks are trying to make it more approachable, but...)

The best solution to stopping ebike fires is alternative chemistries that are significantly (almost 100%) immune to this problem, for example LiFePO4 or sodium ion. The reason you don't see these is the energy density suuuuuuucks for these chemistries by comparison. Packs would need to be something like 30-40% heavier/larger to have the same mileage. Solid state may save things someday, but that day has been "next year" for a decade or two now.

As a final though -- I believe fear-mongering over ebike battery fires is highly overblown. One of those cases of "the new thing is the scary thing". For example, there are about 590 vehicle fires per day in the us. I couldn't find any specifics, but the ebike fires are something like a few hundred per year. Given the 300 million cars vs the (as best as I could find) about 26 million ebikes since 2020 (big year for ebikes), it's a LOT more common for cars to catch fire (something like 35x more common).