Why round holes are cheap and square ones are not
How the shape of a feature decides which machine is able to make it.
A round hole is cheap to make because it can be produced by a tool spinning around a fixed centre, which sweeps out a perfect circle almost by accident, while a square hole needs its four sharp corners cut in individually, since nothing spinning around a single centre can ever reach into a corner without also cutting away the material beside it.
A spinning edge can only draw a circle
A drill bit removes material by spinning rapidly around a fixed axis while a sharp edge near its tip cuts into whatever it touches, and because every point on that cutting edge travels in a circle around the same centre, the hole it leaves behind is a circle too. A corner is a place where a boundary suddenly changes direction, and a tool travelling in a smooth circle around a fixed centre has no way to make that sudden turn.
Tying a length of string to a pencil and to a drawing pin, then keeping the string taut and swinging the pencil around the pin, draws a near-perfect circle almost without trying, because the pencil is held a fixed distance from a fixed centre. Drawing a square the same way is impossible, since a square needs the pencil's distance from the centre to keep changing as it goes round the corners. A spinning drill bit is the string and pin made solid: its cutting edge is held a fixed distance from the centre by the shape of the tool, and that alone guarantees a round hole.
The same geometry follows a spinning cutter even when it is moved sideways to carve out a square pocket. A milling cutter ten millimetres across, run around the inside of a square, leaves every inside corner rounded to a five millimetre radius, the radius of the cutter itself, because the edge of the tool can never get closer to the corner than that.
Where the extra cost of a corner comes from
A square hole needs its four sharp direction changes made deliberately, either by a tool that does not spin at all, pressed or punched straight through in the shape required, or by cutting each straight edge and then cleaning out each corner as a separate, controlled step with a file or a non-rotating tool. Every one of those extra motions has to be planned and carried out on its own. A round hole is finished the moment the bit has passed through, and most of the cost of a square one lives in that second stage of slow work on four small corners.
A designer choosing between a round hole and a square one, where either would work, is really choosing between a hole a spinning tool finishes in one pass and one that needs an entirely different process afterwards, and that choice shows up directly in how long the part takes to make. A slot with rounded ends sits between the two, since a spinning cutter can trace its whole outline in one continuous path, which is why rounded slots are so common on parts where a rectangular one would have done the same job.