It's just a matter of convenience.
If a batsman has to tie his own shoelaces, he has to remove his gloves, and also look around his pads. Try wearing pads, kneeling down, and tying your shoelaces. You'll see the pads do seem to get in the way. Also, it's not like the batsman walks very far to ask a fielder/umpire to do this. He only asks the nearest person to him (who can tie his laces without much bother) to help him out, be it a fielder or an umpire.
But its not that a batsman cannot tie his own laces. I have seen plenty of cases where the batsman does it himself (like you mentioned, usually when there is nobody nearby to help). He just asks a fielder/umpire because it is less of a hassle and usually, fielders/umpires are glad to help out. Cricket is said to be the "gentleman's game" after all.