The Shade Tree Mechanic has gone the way of the tune-up these days, there just isn’t much use for either. Automechanics encompasses so many different disciplines you have both the use of hand and power tools you need to know about chemistry and physics and electrical theory then you need to know more than just the names of many complex systems. Figure 2 years of booklearning and practice on “cadaver” vehicles before you (not just you specificaly) are competent to diagnois the problem is the spark plugs and be able to handle any problem that comes up changing them.
This automechanics job is not as easy a many would think and many try when they have no business even trying.