we've done the linear stuff all ready, and some bosses are mathematically impossible without x amount of gear.
If you can get gear from bosses 4 7 and 9, so next week it is possible would you really suggest wiping on bosses 1 2 or 3 for countless weeks, gaining nothing, and not progressing?
Why not take the path of less resistance? (which after all is what you're moaning you can't do!)
Your question isn't why do we, it's why can't you? Simply if you can't clear normal in the sequence it is designed, you either need to be happy clearing it in LFR, or improve.