Mines are meant as a defensive option to a base but the Boom Beach mines have a neon sign over them pointing to the attacker as if to say "Hey, I'm here ... barrage me, critter me, drop an artillery of just run over with me a Scorcher so I'm totally useless". Yes, I know a person could scout a base to drop artillery but artillery wouldn't reveal the "boom" of a mine so the only way to scout for mines would be to run troops (or critter) over them and that costs the attacker resources. Any base can be efficiently defeated by a wide variety of attacking methods and this would provide a form of game balancing.
1. Supercell has addressed this and stated that mines will never be invisible.
2. A lot of times, the purpose of mines is to either prevent a player from going somewhere or forcing them to use GBE to take them out
3. Even if mines where invisible, the problem is that a player can attack multiple times, This means that they can do a pre-attack with weapons like barrage and critters to find where the mines are. Besides making invisibility useless, it would also imbalance the game because for imitation game the player can only use 1-2 attacks when trying to take out all of the bases. However, this again would be unfair because normally players can attack as many times as they want, and therefore find out beforehand where the mines are. This is why in Clash of Clans they are invisible, because a player can only attack once.