Because UI designers only work one phase of the design i.e. they work upon only how the interface has to look for the users where as UX designers works on entire design like they work upon how to provide smooth experience to users while using the interface, so UX designers have more work when compared to UI designer that's the reason they are paid more when compared to UI designers. Even companies are looking for someone who has knowledge on both UI/UX concepts. If you are planning to upskill yourself in this field I recommend you take up UI UX Course from Intellipaat which helps you to master both UI and UX design through a single course.
Before that do check out this UI/UX free course video from Intellipaat
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