Because for him the election is a proxy for another referendum.
His whole mantra is "get Brexit done, and move on".
Swinson's position of "revoke and move on" is a wholly rational response in the context of Johnson's framing.
But apparently what is democracy in action for Johnson is anti-democracy when the Lib Dems react.
We are truly in the "age of unreason".
Prime Minister Boris Johnson speaks to Robert Peston
Some of you will be screaming that "we had a referendum, so the only legitimate way to cancel it is to hold another one".
Except under our unwritten constitution referendums have no legal force, and are certainly inferior in their democratic weight to general elections.
But I don't want to have a fight with you about any of this.
It looks as though the Lib Dems misjudged the mood of the country.
All I said was I feel a bit sorry for Swinson.
As for Johnson, he kept insisting our Brexit agony would be over if he wins the election and his revised Withdrawal Agreement is approved by parliament by the end of January.
He insisted that the nature of our future relationship with the EU, and with the US - as the important corollary - will only be of interest to obsessives like me, and that the rest of the UK will give him the latitude to busk whatever seems convenient. He said that, in terms.
Maybe he is right, that all most of you want is the divorce with the EU to go through in a legal sense, and that you have no interest in what the tapestry will look like formed by the delicate interwoven threads of our future security and commercial relationships with the EU and America and the rest of the world.
Johnson simply dodged questions about what would happen in the plausible scenario that there is no trade deal negotiated with the EU by his deadline of the end of 2020, or how to meet America's strictures for a trade deal when they are incompatible with so many of the EU's.
Johnson is not offering a detailed rubric for the UK's complex and important economic, diplomatic and security ties with our most important allies.
He is simply asking us to trust him, that there won't be a deferred no-deal Brexit with the EU at the end of 2020, or that he will get the balance right between workers' rights, health and safety, the environment and commercial interests when bargaining with the US and the EU.
No compelling vision.
Just a "trust me".
This election is all about how many of us do, and will.