Because using theories to determine what is observable and what isn't leads to contradictions.

Because any "observation" that relies on an instrument, e.g., a microscope or a cloud chamber, is not a genuine observation of something real.

Because issues in the philosophy of science regarding the proper methodology of science are firmly established and relatively unchanging, while the observational-theoretical dichotomy remains subject to much debate.

Because if whether we can observe some entity determines whether it is real or not, then we have items popping into and out of existence depending on our technology. And that's nuts.

Because the observational-theoretical dichotomy bears no resemblance to either a Zamboni or a polar icebreaker, whether of the steam, diesel, or nuclear variety.