Why isn't PCB layout just an exercise in joining the dots?
Design for test - Ensuring the board is suitable for automated test proceedures, and that testpoints can be easily accessed. Making footprints for A.O.T. (alter on test), components robust enough to withstand multiple changes.
Design for manufacture - Circuit boards that work straight off the production line don't happen by accident. Poor component placement and inappropriate track widths, pad sizes and their relative spacings can lead to low yields and expensive re-work, both in time and money.
Design for serviceability - Without consideration for enclosures and the overall product assembly, it may not be possible to access serviceable pcb mounted components.
Design for performance - Failing to recognise signal types, (for example: sensitive or hostile or high speed signals, differential pairs, etc.), or even using unsuitable track widths for the amount of current they will need to carry can be the difference between optimal performance and a board which will not work at all.