Why do they usually make comparators open collector?
The main reason for using open-collector is so that several comparators can have their output connected together in a wired OR gate. All the open collectors can be tied together to a single resistor without any conflicts between comparators. This would not have been easy to do when your comparator has also the capability to source current at its output.

This wired OR gate is useful for detecting ranges of voltage, for example, when you want your output to be low when the signal is below a value or above another value.