His own petition says:

> In the Y Combinator community, one-third of startups with exposure to SVB used SVB as their sole bank account.

So apparently two-thirds of startups in YC with SVB have at least one other bank. That might not be representative of startups as a whole, but is at least a good data point.

Possibly 'bootstrapped' is the key factor here - many non-bootstrapped startups will want to open an account with a very big cheque and little else (from the bank's point of view), and likely will fall foul of Anti Money Laundering policies.

SVB was a major bank and most businesses in the USA only have one bank. Once you have an established relationship it’s just easier to do all of your banking with that bank.

That doesn't answer the question. Why does the government need to underwrite the risk you assume for convenience? And "everybody does it" isn't an excuse, it just makes the prospect of the government doing so all the more expensive to the taxpayer.