Hence "Light" artillery. Not particularly high damage, but cheaper and more common.
If you were a tank (or mech or anything else for that matter) sitting inside a base and LRMs started raining on you, would you really feel like just sitting there even if they didn't do much damage? You'd have to go repair eventually, which would leave you open to a rush. Not to mention the LRM unit would be safely gaining money and rank from the bombardment with little threat from you.
Right now it's a very stark black and white picture. You camp in the base, and you're safe from outside attack. They get a Longtom, boom, you're crippled or dead in an instant. Some indirect area saturation from LRMs would add an intermediate step to this.
