We tell them it's so they'll get the first shot. Which is true, but another reason is that they're always tripping and falling with their guns cocked, and if the guide were in front, he'd get shot.
12 seconds sound clip from the X Minus 1 - A Gun for Dinosaur by L. Sprague de Camp classic radio drama series.
You can hear this line at 00:12:23 in the radio play.
Quote context
[...]
- Well, now, then. First task is fresh meat.
- As I told you, Holtzinger wanted a ceratopsian head. James insisted on a tyrannosaur.
- Then everybody would think that he'd shot the most dangerous game.
- Well, we started off searching for meat. The Raja and I put the sahibs in front.
- Where?
- See? Crouching over there, feeding on those cycads. About the size of a man.
- No, not likely. That bulge on the head is solid bone.
- Now, then, hold on there, James. You've had your shot for the day.
- Hold your fire until Holtzinger shoots.
- All right, go ahead, Mr. Holtzinger.
- No. Here. Try that one by the rock. There's a good clear shot.
[...]