I assume that when you say stuff you mean media files. Well the reason is that a media file is a collection of things. For instance most media frameworks and players do their own demuxers (as using library versions makes things like trick modes hard to do) and the demuxer is more often than the decoder the one which has to battle with weird files. The second differentation factor is crash policy. The more broken files your player tries to play, the easier it is for said player to encounter something that makes it crash. This is a security risk. So as a player developer one are made to make a decision on how strict to be with more strict meaning less crashes but also less files being playable.