>>/25878/
> Poles had sejm
Yes they did. In Poland the assembly of the estates was very powerful. Too powerful. Other European countries had similar institution and many had their fair share turning the tables (like in the 17th century England).
> lack central authority
All is about the movement and balance of power. Who practiced it. In medieval, late medieval (early modern era) societies autocracy pressed against oligarchy - to refer another thread. Kings tried to claim the right of making decisions to themselves while nobility and clergy (and some extent commoners) did the same. But oligarchy was a rarely united front, chiefly two oligarchic parties pressed against each other in the fight for power, sometimes kings used one party for their advantage, sometimes one party used the king for their advantage, sometimes both parties had their king who used them, or they the kings.
Poland is a great example for this chaotic situation.
For your RE example: on the Hungary election was there from the formation of Hungarians, when the tribes made the Blood Pact during the late 9th century. When there was a stable royal family - at the times of Árpáds and Habsburgs - it was a formality, but between these two dynasties other houses changed rapidly after a couple of kings and election was taken seriously.