Is it really, you yourself said that they do exactly that
I said they do exactly what? All gouge folks?
Hardly; I work in the industry and support the majority. There are of course dodgies - just like in every industry.
And, sometimes they diagnose wrong. Simple as that. Everyone is human, and faults can be tough to get to the bottom of quickly.
Sounds like replacing every part in a process of elimination to me.
Yes; as I said.
This will hopefully get to the cause cheaply for the customer. If the first possibility doesn't work; then you move to the next possibility. That's all you can do.
Many issues with cars can show a symptom with multiple causes of the symptom.
Noone likes to spend money on their cars, and most good mechanics realise this, so try to work from the bottom up in terms of cost and time (also cost).
For eg; a car that is misfiring - can be a blocked fuel filter (which also leads to a weak/failing fuel pump), a dodgy spark plug and/or spark plug leads, leaking valve gasket into the spark plugs, even blocked/disintegrating interior of catalytic converters, broken crankshaft, and so on etc.
In the case of cars with iridium spark plugs; often it takes at least half an hour just to check one, and/or the lead that is attached to it/them, and/or the valve gasket leak..
Where do you start?
If the fuel pump tests ok, then the obvious start is the fuel filter (if it is external) - cheap and easy. If it doesn't work; possibly fuel pump or move to the spark plugs/leads.
I would have thought the cheapest method may be to do a test to find where there is resistance in the circuit but all anyone want to to is replace batteries, starter motors etc
The first test for any mechanic would be to test the battery, then the alternator.
The starter motor would display a similar symptom to a dead battery - no crank at all....but it is usually more expensive than a battery, and the battery test would indicate the likely option; most times a no crank with a good battery would l;ead you to assume a possible starter motor.
Hence; start with battery and alternator test. If these are ok, then it could be starter motor, or some internal wiring/drain issue - time for an auto-electrician.
But, it can also depend on the age of the battery, the usage pattern of the car - short, infrequent trips will shorten the life of a battery.
The battery may be the wrong one for the car - sometimes when folks are stranded and need to get going asap, the roadside crew will install the closest compatible battery if they don't have the right one. They have no choice; the customer wants to get going NOW.
This will add to the complications and shorten the life of the battery, the alternator and possibly other things.
Mechanics can't win; start with the dearest option and get slammed for a gouge, or start with the cheapest and easiest option and slammed for a gouge of a different nature.
Please don't bring your car to my workshop.