It's not a requirement. It's a recommendation by CAA/EASA/OSD, sooooo if you are unfortunate to have an accident with a solo student and he only has, say, 15hrs they may ask some awkward questions.
I suppose it's about risk mitigation and YOU reducing risk to as low as possible.
References:
OSD
SFAR 73
AIC P066/2013
Hope this helps 👍