I fixed a parasitic drain on my car in 408 days

These things are not engineered to the same requirements as the OEM electronics, and if they are drawing 10s of milliamps while the car is off, that is certainly enough that you wouldn't notice an issue with a newer batter and a car being driven every day. But if your driving patterns change and the battery gets older, suddenly your battery is dying all the time and "nothing has changed."

The design of most cars can't tolerate losing 12V DC power without problems. Clearing superfluous codes after a battery swap is SOP. Cars are build for two simultaneous customers: regulatory regimes and work-a-day 13.5K miles a year workers that start (and thus charge) their cars at least every 48-72 hours. Other use cases aren't really considered.

Some are better than others though. I have a 21 year old Nissan with a factory alarm and key fobs that sits for a month or more and starts fine every time. Heating helps a lot.

Having worked as a mechanic in my youth, and worked as a software engineer in SV, the kinds of troubleshooting skills and detail-oriented attention span involved in diagnosing these issues are far more common in the latter than the former industries.

My impression is it's rare for someone with such abilities to stay a mechanic, they can earn far more money in tech, with less exposure to hazards.

On the subject of mechanics failing to diagnose electrical issues, I have my own story too:

Decades ago, back in IL, a friend inherited a low-mileage minimalist Ford Escort hatchback, manual trans, crank windows, it was a great little econobox to inherit, on paper. He kept having the battery die on him. Not being a mechanic or even a hobbyist gearhead himself, he kept bringing it to shops. They replaced the alternator, the wiring harness, the battery, the starter, they just kept throwing parts at the car. This is what most "mechanics" do nowadays; a poorly informed process of elimination via new parts, on your dime.

I hadn't been in contact with this friend for years when I heard about this "cursed" low-mileage car sitting in his garage, full of new parts with invoices totaling well over $1k. He was car-less at the time because of this situation. I offered to fix it for him, but he didn't have any confidence left in the vehicle or my ability to fix it, he was understandably fatigued by the whole thing. So I offered something like $250 and took the car off his hands.

15 minutes with a voltmeter revealed a huge voltage drop across the negative battery terminal and the chassis. Followed the negative strap to where it attached to the chassis and the area was corroded (recall it's an IL car). Removed the rusty bolt, wire-brushed the unibody steel behind it, the bolt and cable lug, slapped dielectric grease on everything and reassembled.

The car charged the battery fine and ran like a champ. I ended up selling it back to him a year or two later for basically what I paid plus a few hundred for my trouble/towing etc. He wouldn't even take it back until I had driven it for years to prove it was fixed. It was that brutal an experience for him, dealing with "expert" "mechanics" bleeding him dry.