I have regularly seen people not recommending plain soil for pots, since the roots of the newly generated seedlings suffocate in the lack of oxygen due to its compact nature and lack of water drainage. Yet, I have seen people here propagate marigold and holy basil (Ocimum sanctum) pretty much fine in plain soil. The plants also turn out pretty healthy. Same goes with aloe vera. How is that?
Firstly I assume with "plain soil" you mean dug up from outside in contrast to a bought or self-made potting mix.
Secondly a recommendation is most often just a best practice. I've seen many people do a lot of stuff I wouldn't recommend doing and do fine. That doesn't mean that there isn't a reason for the recommendation.
So for containers the main problems are constricted space. That's why potting mix exists. It's usually pretty rich in nutrients and has decent water retention while still allowing for good drainage.
So what's the difference between the soil from your garden and potting mix? The main constituent of the "plain soil" are minerals. From large grain to fine they are grouped as sand, silt and clay. Sand drains pretty well but doesn't retain water and nutrients very well, clay retains water and nutrients very well but doesn't drain well. Silt sits somewhere in the middle. Then there is organic matter (humus), water and air. The amounts of salt, silt and clay determines how easy your soil drains and how good it retains nutrients and water. That's where the distinction between heavy and light soils comes from.
For potting mix the mineral part is replaced by other things like peat moss, coco peat, vermiculite, perlite or sand. Depending on the recipe of the specific manufacturer. There is usually more organic matter in there than plain soil too, to solve the nutrient problem (plants in the field can root deeper and wider that in a pot so the soil doesn't need to be as nutrient dense to provide the same amount to the plant).
So if you got very sandy soil with no clay the only thing you would only need to solve the nutrient problem - with some additional compost for example - to get a usable potting mix. I got a sandy clay in my garden. Nice to work with in the garden but if I put it in a pot I get a rock-hard lump of clay soil after a few days. So I wouldn't much recommend using that in a pot.
Thus, the recommendation to use potting mix instead of soil. It saves a lot of hassle and your soil is probably not as light as you think at a first glance. But it isn't impossible to pot with plain soil.
Now for nursing seedlings to be transplanted later there is an addition consideration: You usually use something with very few nutrients (pure coco peat for example). The seed contains everything needed to get the seedling going and the sparse nutrition forces the plant to send out lots of roots.
So just because something is recommended doesn't mean it's impossible to do it any other way. But it usually is easier amd less error prone.
The biggest reason you don't use ordinary garden soil in pots is because of possible pathogens - there are many pathogens in open soil which aren't a problem there, but might well be so contained in a pot. The same is true of compost which has not been produced using an aerobic, hot method of composting. Most people's compost is made anaerobically and it does not get hot enough to kill off any pathogens which may be present, so it is very useful as a soil conditioner on open ground, but not suitable for containers. Then there are the other reasons not to use garden soil, like it has too high a clay content or its too sandy or whatever...
For striking cuttings and growing seeds, it's recommended to use starter or seed and cutting potting soil, with the seedlings or cuttings transferred when they are big enough to either potting soil in containers or into the ground. Clearly, many people do risk using ordinary soil and it works out, most of the time they get away with it, but results can be variable and may be disastrous.