Because every single system of the truck is going to be wearing out by this point. What is a mechanic going to charge? 60-100/hr at least? It would make sense to buy a newer truck that needs less if you have to pay for labor. Its why people do what they do and why old cars are cheap - the market itself is telling you this.
I like my old ford. Go ahead and buy it. Expect to continually have to repair it (which will be easy because it is simple). Everything from heater cores corroding though to connectors burning up to switches breaking, lights burning out, door hinges wearing, windshield cracking, crank no start, bad batteries, alternator, belts, etc. All that is on top of oil leaks from worn seals, and major systems wearing out, like engine, transmission, driveshaft universal joints, and the rear end. Also brakes.