[inglor_cz]

Historical reasons. For 40 years, the continent was split right in the middle. Back then, it was absolutely natural to speak about the Western Bloc and the Eastern Bloc, possibly with neutral Austria and Switzerland as the weird exceptions.

Of course there are huge differences between, say, Czechia and Moldova, but we were part of a single allied bloc controlled by a single power for two generations, so there are some similarities as well. You can see some of them until today in architecture etc.

[gambiting]

I mean I'm Polish, and perhaps wrongly, but I have never thought about myself as Eastern European - for me that means Belarussians, Ukrainians, Romanians......for me "central European" is completely the right term for Poland, Slovakia and Czechia, but I understand it might not be the commonly accepted view.

[inglor_cz]

I do not feel particularly Eastern either, but it is true that the former cohesion of Central Europe was broken into two by running completely different economic systems on parts of it. Bavaria, Czechia, Saxony and Austria used to be fairly tightly connected before 1945, now the differences between wealth of the formerly Western and formerly Eastern parts of the region are massive.