It's not an absolute rule as I understand it but a pragmatic risk balance. The still birth rate in normal pregnancy rises after 40 weeks. After 42 weeks it rises even more, which is why 42 weeks is the usual cut off for induction. With IVF pregnancies the rates seem to rise at the same sort of dates, but even more steeply. Hence a lot of obstetricians not wanting ivf to go past term. No-one seems to know why this is the case, might be because the women pregnant with IVF tend to be a bit older (have spent years trying, fertility declines with age) or because there's something tied up with the whole fertility thing that necessitates the IVF in the first place. Either way, the general stillbirth risk post 40 weeks is higher than a non IVF pregnancy. There's no way of predicting if you will be 'high' or 'low' risk on that curve. So frequently, it's recommended that getting LO out as soon as they're cooked is a pragmatic thing to do (and with IVF there's no issue about exact dating). Yes, an induction sounds potentially pretty miserable and increases the chances of further interventions being needed (syntocinon infusion, epidurals, section etc) but..... You might well end up with these anyway and you might consider it a small price to pay to decrease the other risk. It's a balance, with no definite answer, as is so much of medicine. You don't have to say yes but do ask your HCP why they recommend it.
I am somewhat grumpily accepting that this is probably the only chance I'm ever going to have to be pregnant/deliver, and my initial ideas of gently labouring at home with telly and pets and dh for distraction til the last moment are probably not going to happen. It'll be pessaries at dawn and the whole works after that. Harrumph. But, almost certainly, a well baby at the end of it.
Depends what your own risk tolerance is really.