I've always wondered why most mech mods have the fire button on the bottom. It would be more "natural" for it to be on the side of the tube
it is simply easier for them to make it that way
That's only a side effect.
As a primary reason I'd say the switch needs to be able to take an arseload of current which means it will have some heft built into it. Having such a switch on the side of a mod would look awkward. If you look at mech boxes the switch is sleek on the outside but either doesn't actually conduct the current or is considerably more massive inside.
You either have to abandon the tube style and make some sort of bump/hump (which likely will always catch when you try to get it out of your pocket), make the tube a hilarious diameter for the battery it carries or do what the greek mod makers used to do: make the mod longer to house an internal mechanism (the switch still sticks out some).
So it mainly comes down to space efficiency for the massive switch a mech needs.
Coincidentally it's also easier to manufacture the .... switch (as we call it in Germany)
As to the OP, the options for vapenises have pretty much been laid out
high end -> dicodes
lower end -> uwell nunchaku or ehpro 101 (which I believe is discontinued so may be hard to find)