Probably because you will get little credit against the 15 hours training required once you get back to EASA. However, there are times when being dual qualified with EASA and FAA are beneficial.
The other point you need to bear in mind is that it is perfectly legal to “dual count” hours in the FAA system in that if you are rated for the aircraft and undergoing training you can log that training as P1. When you come back to EASA your 100hours P1 requirement for CPL issue may not be accepted meaning you having to go and do some more P1 time.
That said, still worth doing the IR and, IMHO, the CPL as well for the experience - but don’t waste hours doing circuits or local burger runs - go places and challenge yourself.