This is caused by a few factors:
- Modern farm maps are relatively easy for most of the map with a large but short spike in difficulty at the beginning or the end, so you only need to play hard stuff for a short time. The hard part makes the map worth a lot of pp, while the rest of the map being easy lets you get a high combo fairly easily, so even if you miss in the hard part, you'll still get a lot of pp. An update to the pp system that aims to fix this is in development right now.
- Farm maps are also designed in a way that makes them only test the player's raw aim instead of other skillsets: The vast majority of rhythm gaps between objects are half a beat long, because faster and/or more varied rhythms would test the player's reading and finger control. As for movement, the constant spacing and sharp angles make snap aim as comfortable as possible, since more variation in spacing and angles takes more aim control to hit. The AR is also high enough that cursor awareness is a very small factor and the number of objects on the screen at once doesn't challenge reading. The current pp system is horribly lacking in this regard; variation isn't considered at all, pp just scales with rhythm density and spacing as well as the width of the angles between objects, which is a relatively recent addition.
- These maps also have a fairly high OD value. It pretty much just makes any play worth more since you get more pp for accuracy, but it isn't actually much of a challenge compared to many other maps because of the simple rhythm I mentioned earlier.
tldr they're finetuned to be as comfortable to play as possible and to abuse certain aspects of the pp algorithm to give lots of pp, but the pp system is also just horribly biased in favour of these kinds of maps