The fact that doctor and dentist can’t tell you why the teeth are like this is probably a good thing: it means that there is no “bad reason” for it, and nothing needs to be done to fix it.
When both of a pair of teeth is affected (i.e. the two inside bottom teeth, or the two upper fang teeth) that usually means that something happened to these teeth earlier in their development. This is because teeth develop in pairs. What probably happened is this: At a certain point, these two teeth were in a stage of development that left them vulnerable to stress, like a pregnant mom getting the flu (none of the other teeth were in this particular stage at that time). Only the two vulnerable teeth are affected. We usually don’t worry about this, since the teeth are structurally fine, and they will be falling out in a few years anyway.