Check out the phase diagram for water. You can find it here (right now, anyway):https://en.wikipedia.org/wiki/Phase_diagram
Look at how the interface between solid and liquid changes as a function of pressure compared to the interface between liquid and gas.
An important point is that stating
We do not have to do this for the freezing point.
Is a little misleading to yourself. In principle, you should always do a baseline because the real values for freezing and boiling points of "pure water" can deviate from the idealized (0 and 100 C), and the degree of change when adding solute always is compared against the "pure water" frame of reference. In practice, the freezing point of pure water will only have small deviations from the theoretical value in most normal situations, whereas the boiling point of pure water can deviate by several degrees just based on your elevation... so it's much more important to take a baseline for the boiling point and then you can just assume the freezing point is 0 degrees. Your error by taking the latter assumption will usually be small. That's not the same as "we don't have to do this". As always, it depends on how accurate a measurement you need. But it can't be stressed enough: it's always
better to take a baseline measurement than just assume a value, if only because a baseline also can give you confidence that your method is working properly.