1. when you are working with multiple cars+models it is much harder to scale, every car is its own challenge, as such the operation is likely to remain small
2. customers might expect the rest of the car to also improve which would be problematic when you have to source parts or fix 10 to 15 year old car models
3. every new model you work with is a bit of an experiment, you don't really know whether your configuration is going to cause problems until it does; what worked with another model might not work with this one, in the worst case scenario, people get hurt and you're to blame; if you read any of the 100s of EV conversion blogs, they never get it right the first time round, there's always something that needs rethinking
As a side note, you don't have to buy anything from Tesla to do this. You can furnish yourself a whole package relatively easily from various suppliers. People were doing it before Tesla was even a thing.