The amount of people midwives have to see at the surgery, plus everything else they do in the community, every 4 weeks isn't feasible.
Every 8 weeks is, and if something is likely to go wrong, more often than not you will have more regular appointments, if something does go wrong it is rare that there will be no symptoms or that more regular checks would have prevented it, so there really is no point for midwives to see people more often just to put their mind at ease. To do this we would need to employ more midwives to work in the community base and we don't even have enough midwives in the hospitals so it's not likely to happen.
I'd imagine if the choice was between a m/w visit every 4 weeks and less staff when you are actually in labour or having problmes and in hospital or a visit every 8 weeks and a better number of staff (which often equates to a better standard of care) most people would choose the latter.
I can understand for those who have had mmc that it is incredibly hard not to worry, and for people in this situation, maybe the first sets of antenatal checks should be more regular, until movements can be used as a reassurance, but pregnancy is a normal thing to go through, women do it all the time, we can not be expected to be babysat through it by busy overstretched midwives.
FWIW, I'd happily give up some of my appointments to a woman who had previously miscarried and wanted reassurance and rarely bother my m/w if I have a problem but go straight to the specialist unit I am being dealt with as they have more time and budget to deal with it.