Yes, of course, you could bring your computer but it is not at all the same to carry an heavy computer or to have a smart phone in your pocket. For instance, you can use your phone or your tablet all along the site during a handover to take notes and pictures. It is a lot more difficult to do that with a computer
Your point about tiny, portable cameras is well taken. OTOH, tiny point-and-shoot cameras that fit in a shirt pocket have been around for ages, and are (for the picture quality) far cheaper than a phone.
The Toughbook CF-M34 (introduced no later than 1999... [That's almost a decade and a half ago.]) weighs four pounds (only 4x heavier than a current iPad, or 2.6x heavier than a first-gen iPad) and has a hand-strap [0][1]. Frankly, I'd almost always prefer to type one-handed than to use a software keyboard. If you've work gloves on, you're going to find it really tough to use a capacitive touchscreen. And, if you work in construction, you're probably strong enough to hold a four pound device out at arms length for extended periods. :)
The innovation that modern tablets brought to the world was not to make mobile computing easy, but to make a mobile computer that's nothing but screen. I don't know your personal history, but if you're an early twenty-something, you're likely to be unaware of much of the very useful portable computing that was done and continues to be done (without modern tablets and smartphones!) by folks who do and manage physical labor for a living.
