A formal alliance is a crazy idea.
The talk of the Tories being a party of 40% is misleading as it suggests that there is near 60% waiting to beat them, if only they could commit.
Look at the last few elections. None have had a clear anti-Tory majority.
2010 - The LDs went with the Tories.
2015 - The LDs went into the election saying they would continue the coalition. In the end they weren't needed, and the Tory+UKIP+Unionists was over 50% of the vote.
2017 - The LDs specifically rules out supporting Corbyn, but refused to rule out supporting a Tory leader.
2019 - Possibly the only time that the obvious candidates for an alliance were anti-Tory, and still you're looking at around 48% for the Tories+Brexit+Unionists, which, with FPTP was still likely close enough to push them over even with a Lib/Lab pact. We also had talk before the election of EuroTories helping form a govt of national unity, but it was made clear that the LDs and SNP wouldn't do it with Corbyn in charge, so clearly they weren't all going to cosy up after an election.
So, even with the current situation, there hasn't been an obvious and easy anti-Tory alliance that we could have put together. BUT it's worse than that, as an alliance would only cut the votes for the parties involved.
Why would a Tory leaning LD vote for them in a formal alliance that brings a Labour led govt closer? We saw in 2015 that well over a million 2010 LD voters, went to the Tories.
Bizarre as it may seem to us, between 2017 and 2019 1.5million voters defected from Labour direct to the Tories. There are plenty of Labour voters who would rather vote Tory than have the SNP or Greens in positions of power.
It's a really lovely idea to think that the Tories are hated by most of the population, but sadly it's not true, and a formal alliance makes a Tory majority more likely.
What's happening now, with Labour and the Lib Dems having a soft alliance, and Starmer avoiding alienating LD voters is the best way to win an election.