Earlier quoted context omitted.
All miles are not created equals though. Sure Tesla will gather a lot of data quickly but most of this data will be from people who use their car for their daily commute, everyday at roughly the same hour. This doesn't have the same value as data from different areas/road conditions/time of day
The USA is a big country with a lot of variety though. Different timezones, climates, terrain. It does provide some coverage I would think.
And should there be places, terrains, urban conditions etc. that Tesla doesn't cover, they will see it and can hire a number of chauffeurs to drive around there. Even if they hired 1,000 drivers to gather data in certain geographical areas at certain hours or weather conditions that they lacked input from it would be a drop in the ocean compared with the cost other car manufacturers face to keep up with Tesla now.
You can't just install autonomous driving systems in any combustion engine car without changing lots of systems. Tesla is electric fist. Everything is already wired, ready to be measured and in feedback loops.