Adding in some feature thoughts that haven't been implemented yet too, like Artillary which I'm guessing won't need to be targeted, can also be used to take down walls without making the walls targetable.
Targetable walls, like Anni points out, are only as strong as the one tile that can be dps'd down. When they aren't targetable, then that makes them much more complicated to breach.
From a miner's perspective, a detector bot could previously make the rounds on 4 or 5 beta Island epi spots in less than an hour. Meaning it was very difficult to mine on beta without getting spotted, that includes island owners not just ninja miners. This is also true for the more common resources too, and a reason why people are saying they just mine on Alpha.
Walls make it much more difficult to roam around quickly.
From the roamers perspective, there are normally only about 10 players out on the beta islands, and it takes an hour of roaming just to find one.
Now we have a chicken/egg issue. If walls increase the number of players using beta, then the roamers will have a better chance of finding them; Ratters as well as miners. however, until players realize walls make beta safer, they won't go out and use them, making less targets, and roamers will get frustrated.
Its one thing to know there are targets in an area, then bring in bombs to get to them, and another to expect roamers to carry bombs for recon. But allowing roamers to blow holes in walls, means that sense of security disappears again, and we're back to 10 people in beta.