Adding a perspective from Germany, where the tax code is definitely not simple, a major difference I see is that filing is optional for the simple cases because you can only ever get money back. There are some default deductibles already applied to your payroll tax so the tax office doesn't have to deal with super small cases. The big advantage is that Joe Average won't risk getting into a lot of trouble for not filing.
Just to illustrate, let's go through your examples and how it'd work in Germany:
> side-hustle contracting (IRS doesn't know what expenditures are for the business)
You'll have to mandatory file income taxes since the income from contracting is not salary and there are no payroll taxes deducted from it. Not sure how common it is in the US but in Germany the vast majority of people don't have side hustles like this (for a variety of reasons; certainly a bad thing)
> stock sales (your broker may not know your basis)
There's a default tax rate on capital gains (25%) with a 800€ allowance. You assign how you want to split the allowance between your various banks and other capital gains generating accounts (you're responsibility to not exceed them) and the banks will report your cap gains with your tax ID to the tax office. If you did pay taxes it's often worth filing to make sure the allowance evens out. Also, if you want to carry forward a loss you have to file (but you got 5 years to do so)
> home improvements eligible for tax deductions
You'll probably want to file but you don't _have to_. You just won't get the deduction.
> sold a home (IRS won't know your basis or selling price)
If it was the home you lived in, you don't have to file (it's tax free). If it was a house you rented you'll have to file but you'll have to do that anyway for the rental income.
> did you move for a job? IRS won't know whether you are eligible for tax deductions.
Same as with the other deductions, it's in your best interest to file but you don't have to. No (legal) consequences if you don't.
> crypto gains/losses
This gets tricky but if you owned the coins for more than a year (to the day) they're tax free and you don't have to report them. But you better have documentation on this if you ever get audited.
> inheritances (basis again)
This one is actually interesting as it's a completely separate tax and thus separate process from income tax.
There's an allowance based on your relationship to the deceased (500k€ for spouses, 400k€ for children, etc.), if the inheritance exceeds that you'll get a letter from the tax office asking you to file a declaration for inheritance tax. At that point there's not much software that'll help you and you'll better hire a tax advisor :)