I like this question and hear some version of it almost every day! It shows how important words are in understanding something that is really not so complicated.
A "cataract" IS a lens, just an aged and dysfunctional one. So there is rarely a need to remove it and not replace it with a new, more functional one right at the same time.
Many people do not always realize this was done as an intrinsic part of their cataract operation
But perhaps you are asking why a new Intraocular lens could not be used to replace an older model IOL that may have been used in an earlier cataract operation. While this is technically possible, there has to a compelling reason to do so -the older IOL is usually so sealed in place that it can be very challenging to remove and even more tricky to fix the newer IOL into place.
J.Trevor Woodhams, Chief of Surgery - Woodhams Eye Clinic