Because it isn't cost effective and the costs/risks outweigh the benefits in a healthy, low risk pregnancy. Basically, there's no advantage in a healthy, low risk pregnancy to routine scans. Babies don't get heart problems or kidney problems, etc. randomly one week and not another. If there is an anomaly, it is picked up at the 12 or 20 week scan. It doesn't usually just magically appear some other random time and when problems do crop up at other times, they have other symptoms - heart rate abnormalities, changes in movement patterns, manifest in symptoms for mum, etc. There's no reason to have a scan to check baby's heart rate, growth, movement or positioning in most cases. These can be checked perfectly well in a normal midwife exam. If they are monitoring something specific, it can be helpful. But there's no added benefit to just checking for the sake of checking.
But there can be costs and risks. For instance, costs to mum to have to take off work to attend hospital scans. Not everyone gets paid time off for appts. I'm self-employed, so I don't. It's lost income and I have to work extra hours in the evening. I'm with a midwife team where all appts are done at home, so midwife comes to my house and does everything. It's only 20 minutes out of my work day, which isn't a big deal. But driving to the hospital is 30 minutes each way, plus probably an hour waiting for scan/having it. I wouldn't want to do that and would be declining them. But then there's also costs to the NHS. There's no reason to be spending money when it doesn't improve health outcomes. That money is better spent on other things than people just wanting to have a look at their babies. Like better continuity of care in midwifery, or improvements to SCBU services for babies who are born poorly and need extra care, etc.
But then there is also the risk of unnecessary intervention. If scans happen at every appt, then there is a greater chance of seeing 'something' that isn't really anything (and wasn't picked up at 12 or 20 week scan), but then intervening on it nonetheless. Some of these unnecessary interventions can produce more harm than good on healthy mums and babies. You see this in the U.S. where routine scans are more the norm (as are other interventions we don't do routinely in the UK). It's part of the reason the U.S. has the worst outcomes in maternal and infant death and illness in the developed world (lack of universal health care and poverty being factors as well, of course). More care isn't always better and the NHS has really strict processes for determining based on the available evidence base what care to offer broadly and what care to reserve only when risk necessitates it.