Ender 3, why only 3 limit switches?
You only need 3 switches to determine/fix the position of the carriage (carrying the hotend) with respect to the printer frame (for X, Y and Z i.e. 3 dimensions). Firmware setup, by defining the offset and the traveling distance on each axis, determines the minimum and maximum displacement. By default the printer carriage will not go outside the boundaries and will not damage anything. That is sufficient for most printers, but, for those printers that have weak steppers and printers that produce prints that suffer from layer shifting, the coordinate system might be messed up, and the carriage may travel outside the initial boundaries, this can damage your printer, using maximum endstops will prevent the printer from destroying itself.
Why use only 3 switches? The reason for this could also be related to costs, you need half the amount of switches, cables and a less extended printer controller board.

I've had one printer setup with max limit switches, but they never got triggered (unless self triggered for testing), never had a layer shifting either.