Regular cab pickups exist and they get reasonable mileage these days. You can get combined 22mpg in an F-150 2.7L v6. It's a bigger truck because it is designed to tow 5000lbs and carry 2000lbs in the bed in its smallest configuration. If you launch boats or operate on wet construction sites, you need 4x4. Why? Your rear wheels will be on a slippery boat ramp. If you don't have 4x4, your rear-wheel drive truck's front wheels are useless and your truck can become a submarine by sliding into water.
It is difficult to buy a new truck with an 8' bed that's not a full-size truck. You need to in general look for a fleet vehicle. However, vans exist. My Grand Caravan minivan converts into a cargo van and fits 4x8 with gate closed. It's dead at the moment and so I got an F-150 to tow it home and drive around until I fix it.
So, if you want an 8' bed in a pickup, you are getting a minimum of a 1/2 ton truck. If you don't need one that long, 6' beds are available on smaller trucks. Your other option is to trailer something since they have 3500lbs towing capacity. 6' bed is perfectly usable for 8' loads with a bed extender.
So, why can't you get an 8' bed in a smaller truck? They are prone to getting overloaded. There's the legend of the immortal Ford Ranger that work much harder than they should. 20 sheets of 3/4" plywood is 1200lbs and that will fit inside a truck bed. Unless you have that 1800-2000lbs capacity, you'll overload it. Now, consider the weight of drywall, cement etc... Anything related to 4x8' items can unintentionally overload a small truck's rated payload capacity relatively easily.
Do you need an off-road 4x4 3/4 ton truck, which are huge and tall, for city driving? You do not. I need my 1-ton truck for towing a car trailer weighing more than 5000lbs. However, it's only driven when when I have to tow something that heavy.