Almost all of them started as elective, technically speaking; it's simply that it's far enough back that it tends to be a touch difficult to piece together all the fine details. For instance, there's the Saxon (and English) Witan, Nordic Things, or Bohemian Estates. Even the Kings of France were elected in the days of Pepin and Charles the Great. The Holy Roman Empire gradually dissolved rather than breaking in one dramatic event, but it too was always at least nominally elective, and its Early Modern weakness is largely due not to hereditary rule, but rather to the Thirty Years War and the victory of the Protestant princes over the Emperor. Similarly, its medieval weakness largely came about due to the great interregnum where there simply was no Emperor, properly speaking. Most often, succession in non-hereditary periods was more likely to come at point of sword than during hereditary periods, where as you note, ties of blood started to create their own tradition supplementing or replacing electoral traditions that effectively either disappeared entirely or was reduced to a rubber-stamp.
Interestingly, I would argue the converse for Poland in that it weakened once the hereditary dynasties (particularly the Jagiellonians) died out and they were reduced to a succession of elective monarchs who relied upon the great magnates for their power and authority, and said magnates conversely were more interested in electing weak monarchs who would not threaten their own authority. At least one of the incidents I outlined actually took place well after the end of hereditary rule in Polish elections. To wit, the one explicitly stated was the first ascension of Augustus the Strong at the expense of the then-Prince of Conti, François Louis (who was a bit of an inveterate philanderer; a particular affair of the heart is blamed for his lukewarm interest in the Polish crown as it would remove him from both France and his wife's sister-in-law's arms). After the last great Jagiellonian king, you have a Valois (ran away for the French throne), a Bathory (elected due to his Jagiellonian wife and co-ruler), a few Vasas, and assorted others. The only ones who came close to establish any sort of dynastic rule after that point were the Vasas and Wettins, and even the latter are kind of skating by on technicalities (non-consecutive rulers, and one of the two had to be restored to the throne by the Russians).
This, by the bye, should be a corollary to the above, and suggest what happens when you give someone absolute rule, even by elective process. Sooner or later, someone's going to pull a White Mountain and crush your puny estates like the Habsburgs did, or else your estates will become dominated by vested interests and fracture as in Poland. Or worse, you will end up with a fratricidal war when a second son asks why he wasn't elected instead of a first son, or a first son passed over in favor of a second son, or a whole mess of cousins with competing claims. Do remember that in World War 1, all of the dynastic rulers of Europe were related by blood, and many had the power to stop the war. Albeit, not without cost, but that's another story...