Yes I also think Czechs avoid Poland
Czechs don't avoid Poland, but they tend to turn up in Wrocław and Katowice simply because it's where the work is and it's close to home. Wrocław is full of them.
The people living in the Czech Republic might not think too highly of Poles because of history - 2 (or 3, depending on viewpoint) invasions in 50 years tends to solidify the idea of Poland being a bit of a bully.
suprised at how Warsaw was developed and that the differences between the both countries seem minimal
Poland is about the same as the Czech Republic these days, it's only the massive rural farming sector that brings Poland down - the Czechs have far less of it. If Poland also didn't have it, then Poland would be miles ahead these days. You only need to look at the amount of construction in Prague and Brno vs Warsaw and Wrocław to see the difference.
The Czech Republic started well ahead of Poland, but it has stagnated badly in the last 15 years or so. It's because of political problems - for instance, they just cannot get roads built there, even though EU money was available.
Anyways with all these Czech speaking postings, I am wondering who they are finding to fill these jobs?
Czechs, Slovaks and Poles, mostly. I know quite a few Poles that easily got to B2/C1 in Czech without much effort. These jobs don't really require absolute fluency in Czech, so making the occasional "Polish" mistake in Czech isn't a big deal.