In cases where fault tolerance isn't as robust, it's for the same reasons as other disciplines you mentioned: budget and importance.
My impression from friends working in other engineering disciplines is that software engineering works similarly to other fields: the more risk to human lives is involved, the more testing, redundancy, etc is involved.
Standardisation - in the big 'E' Engineering world, there would be a recognised international standard for Web Apps that ensured/enforced that all Web Apps supported this functionality, or they would not be approved for use.
Another factor is Accountability. A senior Software 'Engineer' would have to take personal responsibility (liability, accountability) that the software product they are producing and/or overseeing met all these requirements and personally sign off that these standards have been met. If the product were to fail at any point and it was determined that the cause was negligence in following the standard, any damages sought (not common, but not unheard of) would ultimately find their way to the accountable individual and their insurance.
In cases where budgets/importance don't allow for this level of scrutiny, there would still be paperwork signed by the producer of the software and the client acknowledging deviation from the standard and waiving any recourse for doing so.
there is totally standardization. At the building block level. TCP/IP, Protocols on top of that, language standards etc.
Web Apps are complex, why would there be a standard? Just like there's no standard for cars, other than for some of their components like wheels or headlights.
See the Australian Design Rules (which happens to form the basis of most UNECE and Canadian transport regulations) if you want to see how detailed they are.
To continue the analogy from earlier - standards wouldn't mean all web applications would have to be designed, programmed and work exactly the same way, but it would mean that they would need to be formally tested (to an approved test plan), and to use your example, would need to demonstrate that each of those layers of fallbacks (as dictated by the standard and covered in the test plan) operate correctly in order to be certified.
If anything, I think software has a huge advantage over physical world engineering in that testing can be replicated at virtually no cost whenever a change is made to the design. I shudder to think how many cars get trashed in order to meet vehicle safety testing requirements.
Here is the Australian Standard for Caravan and light trailer towing components, Part 1: Towbars and towing brackets
https://store.standards.org.au/product/as-4177-1-2004
There are thousands of these documents covering everything to do with transport from the vehicles to the reflectivity of street signs.
The regulation (at least in my state) is that only engineers who are registered as Registered Engineers are permitted to carry out professional engineering services in this state.