tl;dr several reasons:

1. the church considers the reproductive faculty to be exclusively for marriage in the marital act. The marital act must be "procreative and unitive" to be licit, and IVF (and a variety of other fertility processes) frustrate the unitive nature.

2. IVF as currently performed always ends up producing multiple embryos.

2a. The embryos are genetically tested and those that don't make "the cut" (whatever that might be, right now; the goalposts can move and can depend on the parents or various other things) are simply discarded/destroyed, and the church holds that this is infanticide/murder.

2b. The embryos that make the cut but are "extra" go into a freezer, but the church holds that these are human beings that should be accorded the dignity of any other person. They might be used at a later date, they might not.

Even if there were a technique developed (which development the church would also consider illicit because the research would require consuming more embryos) that resulted in exactly zero "extra" embryos, point 1 above still stands.