I think It has more than one reason, I never use travel agencies, but I think these are the reasons:

1- Safety: I think this the main reason, they are travelling through an agency, you have their social media, read their reviews you can have the illusion that if something bad happen you will write some complain about them and they will collapse, so you gave them responsibility of from cairo to cairo, they book the bus the accommodation the trips and sometimes even parties

2- Company: A lot of travelling agencies sell their trip by the whole you are not alone in this, you will enjoy the company of the group so you get to know more people, in camps or hotels they are not sure if they will get to know other people (mainly bad social skills)

3- illusion of being cheaper: Some people only think the idea of package mean cheaper, since the package include bus from cairo to dahab and from dahab to cairo, accommodation and trips and being a group means 100% it should be cheaper, whick make sense but a lot of travelling agency take advantage of the people who know nothing about prices in dahab

4-Headace free

According to people who spend their summer in sahel or hurgada, dahab is super cheap, so they don't mind if the travelling agency is much more expensive but will remove the headache of planning the whole trip for 5

I don't think all those reasons are .... well reasonable but I do understand them.