Why don't they want to be rescued? Are they not in danger?
Is it that they only want help once in UK waters?
You're asking the wrong question.
What would be the point in spending a long cold night hiding in the dunes, boarding a small RIB in the middle of winter for which you've forked maybe thousands of dosh, that is overcrowded, to risk a hazardous Channel crossing, simply to surrender to French border patrol who will return them back to France, only for them to have to hand over another wad of cash for another attempt.
Isn't what illegal, refusing to be rescued because you don't consider yourself to be in danger, then later, in mid-Channel, deciding that you are in danger, and agree to be rescued?
It would be dangerous, and possibly illegal for France to take any refugee boat by force.
You're asking the wrong questions.
The question you should be asking is: why does UK government not make a safe route for asylum seekers?
It would immediately obliterate the 'people smuggling' market, save the RNLI a lot of risk and cash, and demonstrate UK as a civil, moral and ethical country.
If it were not for the RNLI, the UK would be seen internationally as a heartless country, devoid of any care for refugees. Only the actions of the RNLI prevents that reputation for the UK.