Because what you describe is turtling. While you run around and build up defenses and MG nests your opponent has capped the rest of the map and is going to far out tech you. The most effective Brit is a mobile one. Camping/Turtling/SimCity'ing, is easily countered by a good player. As Brits you should:
1) Use your early game strength to your advantage
2) Attack his cutoff if he has one. Use a trench at his cutoff as early as possible since it costs at least 50 munitions to counter. In other words, get up in his face. Dont play passive and lounge around building MG nests and defenses.
3) Build a Mortar if you feel an MG rush coming
4) Get a Bren if you start seeing piospam
5) Get Cromwells and Arty if you see infantry spam or camping
6) Get Fireflies if you see T4.
7) Get a CCS within the first 5 minutes for healing and free Tommies.
Always always keep your officers alive. If your LT dies, get another. Tommies are useless without an LT.
9) Never spam emplacements. It is destined to fail.