It was never a plank during the referendum, when there were repeated assurances that the UK would have an EFTA or similar deal and an orderly, planned withdrawal.
Part of that is because ever since the referendum, every time they've tried to nail down what Brexit actually means in the real world (as May eventually very belated did at the infamous Cabinet retreat at Chequers that led to Johnson and a few others quickly bailing after initially supporting her proposals), it's highlighted how unrealistic and unworkable their plans are and worsened the splits among the pro-Brexit ranks. "No deal" and "WTO Brexit" sound simple and catchy, and are an easy sell to the headbanger rubes.
Any deal short of full EU membership would mean the UK becoming a rule taker rather than having a voice in shaping EU policy and regulations. That was obvious from the start to many of us, but seems to have been slow to dawn on them.
So "no deal" has become the favoured slogan, as if resorting to the mythical WTO trade terms means that the UK wouldn't still have to strike trade deals with the EU in future, but from a far weaker bargaining position, while trying to satisfy the likes of the Trump administration, which is salivating at the prospect of easy pickings.
There's also the factor of EU regulations governing cross-border money laundering and tax havens, which come fully into effect in January next year. Currently, those regulations have become part of UK law, as they had to while the UK was an EU member, and May indicated before her resignation that there were no plans to repeal them. Whether you believed her or not about that, in a Johnson regime, all bets have to be off. Regulatory convergence on such issues is likely to be a key part of any future negotiations with the EU, to avoid the UK undercutting the EU partners, so an ultra-hard "no deal" Brexit is appealing to the big financial players (including some in the Tory Party and Cabinet) who back Brexit.
Also, the more future chaos the better for those who play the money markets - for short-term gain through shorting etc., and because it clears the field for a nice spot of disaster capitalism. For the likes of Dominic Cummings - who's a libertarian in the free market sense and thinks government, and especially the civil service, are long overdue for some culling coupled with a major restructuring of UK society along populist lines - it's an opportunity to set the stage for changes and some sort of revolution I doubt he's even properly thought through himself. But he's going to be alright, having married into the minor landed gentry, so that's nice.
Your final question also highlights a significant factor.
Johnson has built a career on winging it in a grossly irresponsible fashion and never fully reaping the consequences of his cock-ups, and has a massive sense of entitlement and delusions of adequacy. A few years ago, I remember reading an article warning of the consequences if ex-newspaper columnists like Johnson and Gove (let's not grant them the dignity of calling them journalists) got hold of the reins of power. They could make grandiose, sweeping, ill thought out declarations in a column one week, then directly contradict themselves next week if they felt like it, with no serious repercussions. As a journalist, he was sacked twice for lying (lying's a way of life for him, but on those two occasions he overstepped the mark), but still walked into cushy jobs each time. If and when he loses his current job, that pattern will no doubt continue.
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