Because the bombs aren't in phone form-factor, they're big vests packed with nails and shit, just set off by phones. People tend to notice when you walk into the middle of a crowd, drop a large bag, then walk away. Now, in situations where they can pull that off, say crowded busses..., I'm sure they employ that tactic.

Also, I imagine there is a heavier emotional impact associated with 'suicide' bombings.

They are likely being watched by whoever has the detonator, and (reportedly) the bombers often coerce the bomb carriers into participating by threatening their family.

Alternatively, the thought of directly disobeying authority figures never occurs to them. Remember that most of these 'bombers' are women (in societies where women are oppressed) or children.