I think the Boring idea is that at the end of the trip you still have your car. So suburb to suburb works.
> why not just build more advanced subways/underground trains?
The train business is horrible to be in. Just look at Bombardier Transportation/Alstom. City A in State B and country C decides it needs a new train. The existing infrastructure was all custom-made in the 60's. Bombardier/Alstom has to design a mostly custom train for it. Everyone bike shed on the design. Then, because it's a government project, and State B financed some of it there's a clause in the contract that is must be manufactured in State B of Country C. Not only is a new production line (manufacturing hell) for the almost completely custom design required, they must also set shop in this new jurisdiction and work their supply chain around that. Ribbon cutting ceremony, positive polls for reelection.
Few years later, City D in State E of Country Z wants a new train system. The whole process starts all over again. Can't just retool the existing plant in State B of Country C, it has to be a new one too in State E of Country Z, or a local manufacturing partner (ribbon cutting ceremony obliges). Meanwhile the press in State B of Country C gets angry as workers are laid off since there's no more work to be done.
By using standard road vehicles, Musk eliminates one of the biggest cost. It's all off the shelf stuff that's user-supplied.
