Because the Tories have upset everyone internationally, so it isn't really an option. If you've been paying attention the EU has been playing a bunch of jobsworth type games with the UK.

Notice how he will do this in 2025, when the current agreement is up for renewel rather than immediately.

You also have the fact rejoin isn't winding the clock back to 2016, firstly we would loose all of our opt outs, things like the rebate, the euro, etc.. I don't think the reality would actually be popular.

Secondly the UK blocked a number of things like the EU Army (personally I think its a terrible idea, countries that don't spend enough looking to conbine to "save" money) so it isn't the same EU.

Lastly see above mentioned jobsworth behaviour, I would not be surprised if the EU demanded the UK to complete all the paperwork and drag the process out as long as possible (it takes 10 years for most countries).