Because this community fought against open world dungeons. Instance dungeons needs some form of restriction on it or being instanced serves little purpose, you can have 'unlimited instancing' but you also waste resources going that route and a timer is usually a form of allocating your playerbase on those instances.

Usually you do:

-Open world dungeons
-Numerous paths for exploration
-Chests littered around (like any good FF or RPG.)
-Instanced Boss arena.

No problems except people who like having things all to themselves.