Because the flow of the signal is just way nasty.
Seriously, I have crap signal too till I get into a strong signal area. Every HTC phone since the OG evo I've had has had this problem, sometimes worse. So I'm inclined to blame sprint more than I am Sammy on this one. But to sprint's defense, the new network vision promises better everything and is about to go live. I'll reserve judgement till then.