Magic is a “hack” going against the normal orderly rules of the universe. The universe will resist being altered in a strength that grows esponentially with the effects of the anomaly.
If you levitate an apple in a closed room, it has no effect on the rest of the world, so it takes little effort. The effects are local in space and die out in time. After the event, it will be impossible to determine whether it happened at all, so it doesn’t “keep charging” for it.
If you knock over a building, it affects lots of things and has ripple effects in the lives of people affected, contractors who deal with it, etc. If there is a plausible natural explaination the “cost” can be brought down to a workable level of magic expenditure.
Meanwhile, like surface tension, the energy tries to minimise itself, once past the commanded action. Preventing a witness is a tiny bit of magic in itself — on a typical day an ordinary person may look left instead of right, or leave for lunch 5 minutes different, and that change has very litte lasting effect and dies out over time as things are just typical; and it does not seem “unnatural” at all, so it’s very cheap.
If a spectacular miracle got documented in a credible manner, it could be very costly, and it would not happen — his intent to perform magic would fail. But preventing witnesses and even damaging devices that might plausibly break anyway is the soap bubble pulling in to minimise its surface: if the solution can be found that is within the available magic energy, it will work.
The magical beings learn to maximize the effects of their power budget by being subtle, working indirectly, and avoiding witnesses and records.