It's all very well saying wait until the last minute. But it has already been explained in some detail why you're not always allowed onto a train with no crew on it, well in advance of departure time.
Driver who brings the train in really late, may have had an incident on the way in, the train will need an inspection, a OTMR/CCTV/FFCCTV download by a driver manager, a thorough checking over by fitters or inspectors prior to it being brought/put back into service, last thing they want when doing their job is some wittering bunch of passengers complaining why aren't we going anywhere!
Train been delayed in bound, due to signalling difficulties or whatever, it needs to be cleaned & tanked, the last thing cleaners need is some wittering bunch of passengers complaining why aren't we going anywhere & making more mess!
Driver gets on, starts checks, fault found, fitter called, can't be fixed, so you have to go through the rigmarole of chucking everyone off.
Driver gets on, starts checks, all OK, does seemingly the right thing & releases doors, but no guard for train so you have to go through the rigmarole of chucking everyone off.
Crew brings train in leaves doors open & train unattended some nice incontinent pensioner or individual then has an "accident" train has to be removed from service, as it'll need a deep clean.
Crew brings train in leaves doors open & train unattended some nice piece of societies crud comes along etches windows galore, sprays seats with paint & scrawls obscenities on the walls & windows... Train out of service.
There are countless scenarios for this and you know the reasons why we won't allow it, so get over it.