The easy way to ensure that a switch is waterproof is to use something like a magnetic reed switch or hall effect sensor, so the switch just moves a magnet, which is sensed thorugh the plastic case. It uses more real estate than a pure switch, but requires no open holes through the case.
This is at odds with the (apparent) consumer preference of smaller everything.
Um, using a hall effect sensor is exactly the kind of "soft switch" we're trying to avoid, no?
And magnetic reed switches are big, very expensive, vibration sensitive and failure prone. Since this is security, you're going to want it to fail open and you're not going to want a switch that closes briefly if you vibrate the phone.
Mechanical engineering is hard. Let's go shopping! ;)
Reed switches are available that are a few mm long, they aren't exactly huge.
In addition, since switches are an electromechanical component, they also affect your mechanical case engineering which electronic components don't do.
Finally, if your device is "water resistant" or somesuch, you now have an even bigger engineering headache waiting for your design team. Especially since you don't want a pushbutton, you probably want a slide switch.
Switches and buttons are really expensive on electronics components from a relative cost and engineering cost standpoint.