Why arn't all screws the same?

[BigonaBianchi]

I carry a multi tool, its not over heavy, but it's a LOT heavier than one single small allen key. My Headset screw, bar screws, stem clam, seat post, brakes have different allen key screw sizes....i mean the hole you stick your wrench in....so i hav eto carry the multi tool. Why dont manufacturers use screws with the same size allen key adjustments? That way I could carry just one allen key.
Must be a sensible answer or they would, but i cant figure it out.

 

[MontyVeda]

i carry three small allen keys and one longer 8mm one for the cranks, oh and a chain tool... i have a multi tool but most of the allen keys on it aren't required for the bike, and its not a good shape to use, so i never use it for anything, ever.

four allen keys... it's not much.

 

[RWright]

I always thought that the manufacturers must also be investors in the tool companies.

 

[gbb]

They are near enough all M5 and M4. If you were to replace all socket head M5 with button head that might do it for you.

This is true but the downside can be that using a smaller allen key on a larger screw increases the likelihood of ripping the hexagon as you tighten/untighten the screw.
If you do go for button head 5mm screws, get good quality ones and make sure your allen key is in good condition as well.

 

[Levo-Lon]

Different sizes for different torque.
pian I know ,you risk rounding them if you go smaller dia hex heads