I suppose the real reason behind floor plate magazines is that all the early bolt action rifles were based on military rifles which all have their basic design from the late Victorian era, and were replacing single shots. The though process I assume was the cartridge is the disposable bit but the magazines were a fixed part of the rifle and charging of magazine was by a stripper clip. Many early bolt actions also had a magazine cutoff, to allow single loading - theory being that you single loaded, keeping mag full for when a volley was required. Aimed shots were the order of the day.
Even early semi autos - aka the Garand used clips and fixed magazines.
Detacheable magazines came into use with machine guns, and only when the assault rifle / smg cecame widespread were they adopted more widely.
Most at military bolt actions have the means of opening the magazine for cleaning purposes. Mannlichers and Mausers use a bullet to depress a plunger in the floor plate, whereas the enfields have a mag release. The enfields have a detacheable magazine, but they were issued with clips (3up, 2 down) rather than spare mags.