This:
The reason being that Eastern Europe is defined in contrast to Western Europe, much in the same way that Asia is separate from Europe despite both Europe and Asia being clearly part of the same continent - Eurasia. It's basically Western European "exclusiveness" bias (everyone else is seen as somewhat more backwards, even barbarian). So, what ends up happening is that the countries that aren't seen as related to the West are just lumped together.
The only countries that I would say are borderline are Czechia, Hungary and Croatia. Czechia and Hungary could be seen as somewhat Central European rather than Eastern Europe, for the simple reason that they are perceived as more culturally sophisticated (basically, Prague and Budapest), and as a rule Eastern Europe is backwards. And Croatia is often seen as more Mediterranean due to their beach tourism.
And to clear things up, NOBODY sees Poland as part of Central Europe. Whenever anybody mentions Eastern Europe, they think Poland. This isn't necessarily so for Russia, because Russia is seen as more distantly European and, well, Russian. If I were to list the basic regions of Europe as envisioned by Western eyes, it would be as follows: "true" Western Europe (the UK, France, Germany etc.), the Nordic countries (Denmark, Finland, Iceland etc.), the Mediterranean countries (Spain, Portugal, Italy, Greece), Eastern Europe (Poland, Romania, Lithuania etc.), and Russia.