Why doctors and lawyers have better wages than engineers?

Why doctors and lawyers have better wages than engineers? The engineers invented the motorway or highway, trains, cars, motorcycles, airplanes, ships, boats, spacecraft, PC, mobiles, tablets, phablets, freezers, microwaves, mainframes, washmachines, TV, Smart-TV, etc. Hardware and software. If we made their life better, Why they pay less for our Job? This is not fair ! Lawyers' wages: (from 100 to 5000) $/hour, Doctors' wages: (from 80 to 2000) $/hour, 
Engineers' wages: (from 20 to 80) $/hour.
Each Lawyer is specialised in one branch of 13 different categories of law
Each Doctor is specialised in one branch of 35 different human categories of medicine.
But System Engineers specialised in Web Analytics must to handle 50 web analytical tools ranging from forensic analysis, SEO, data extraction, clickstream, multiple outcomes, experimentation, A/B test, MVT, Voice of Customer, Business Intelligence, Competitive Intelligence, reports, dashboards, and Insights.

This not fair !