One of the main problems is that burst damage is way too easily accessible for majorityof champions.

First, an entire mastery tree is dedicated to burst damage (domination tree), and you can get some more burst damage via sorcery tree. Then, we have a lot of items that are "your first auto/spell does extra damage" which creates even more damage, and these items are very cheap. is the worst offender of this, only 2.9k gold is getting BUFFED next patch (3200 -> 2800 gold), stormrazor + duskblade combo is broken because they stack was a massive balance issue pre marksman update, and it still is somewhat of an issue for ap, but it at least has an appropriate price tag This creates teamfights that are extremely frontloaded and usually decided in the first 2 seconds. Sustained damage is rather weak.