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.
Sometimes they are told that they are just transporting and delivering the device to the actual suicide bomber. Then, a spotter sets them up the bomb instead.
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.