Why can't the existing industry do this?It's pretty classic "innovator's dilemma" material but a more interesting comparison here is Fisker (probably going out of business) vs Tesla (currently making a small bit of money).
Fisker did exactly what you suggest, they took a bunch of 'off the shelf' components and bolted them together into a nicely shaped car body. They 'designed' an electric car where "car" was the most significant bit and "electric" was the least significant bit. Since existing off the shelf electric gear couldn't get the range they wanted they added a gas powered generator.
Tesla (and I put all of this on Elon) appear to have ripped up history and started from how do we build something you can drive around based on electricity. They seem to have started with what properties do the batteries have to have, what capacity, what weight does that impose etc. They took that very expensive engineering and thinking about power trains and then slapped a Lotus brand body on it to appeal to the elite geek crowd. And they instrumented the hell out of it.
Then they took all of that knowledge and said, ok, given what we know about how this stuff worked, what did and didn't work? Now lets incorporate that into what comes next.
I found that engineering focused direction very appealing, work from zero, develop an expertise, test that understanding, iterate and improve. Now at rev 2.0 they have a much better product than rev 1.0. Their X series should be really something to behold.
Elon also bet big on being able to create an electric drive train with range. Had he started a couple of years earlier he might have missed out by building with Nickel based batteries rather than Lithium based ones. So there is a timing component as well. The larger companies have all put their toes into the water (the EV-1 for example) but hadn't yet committed to jumping. They don't have to commit since they have a working business, building the equivalent to the Roadster or the Model S would have been merely incremental to their business rather than whether they lived or died. So I expect they were waiting for a bit more clarity on this stuff. They always have the option of buying Tesla later and they take no risk in the meantime. Clearly that was on Toyota's mind when they invested.
You can see the same forces at work in SpaceX so it suggests to me it's a management style that Elon expresses.