Another big reason the center makes the call for the line, and even the QB & RBs, is that he normally leaves the huddle after the first time the play and snap count are called. Thus, he has seen the D's alignment longer, and had more time to fully process what this means to this play in this situation. The center, in the pro-styled Os that most run, is responsible for making the main calls, especially for the entire back-side of the play.
Front-side tackles may make some calls, but they are normall late in the cadence and most of these only effect their guard, TE or RB.
The C will normally call the D&D for the play when he calls the huddle just before the QB gives the call. This can be very important for the QB, WRs, and RBs, who are normally doing a lot of other things between plays, like running on & off the field, getting signals and making sure their arm bands, gloves, socks, and spat still look good n fresh.
The Center should be the "smartest" OLman. By smartest I mean knowing what everyone from one TE to the other is doing on each play. Gaurds can pretty much just do what they're told by either their OT or the C. OTs have to make a lot of decisions pre- and immediatly post-snap. Some teams use calls for this but a lot (maybe most) use hand signals, cause the call is actually telling the RB what to do and they ofter can't hear the OTs anyway.
Centers normally have middle to back side help responsibilities on pure pass (not PlayAction) plays. So upon the snap if the D is playing it straight, the C would jab w/ his play side hand to help that gaurd while looking from the MLB to the backside DT to the Back side OLB to the backside DE. Once this progression is completed, if no one is in need of help, the C should punch enthusiastically at the nearest ribcage he can find. By then he can normally release down field to the ball hoping to knock the last bit of intellegence from the caverns of some defensive tiny-hinny, or fall on a fumble if needed.
The hardest thing for a center to do would be to reach-block a slanted nose on his snap hand with no help. This is a really good way to dislocate your thumb on your snap hand.