Why don't developers prevent players from attacking bases that are 2 or more levels below them... ..

I would suspect that at a certain point, it makes more sense to use your limited number of attacks on higher targets because hitting the low targets don't give as much resources. Second, you can always choose to not play and perhaps if enough people don't play the game because they cannot grow, the rules will change. Third, if changing the rules actually upsets the majority of users because they like attacking much lower level targets then the rules will not change.

Until you go past base level 6 you are protected by the peace shield. this is a good time to build up your stats and then join a decent sized alliance. This will keep you somewhat safer than going it on your own.