This is true. However, in a professional environment, it is reasonable to expect that a professional level of attention and diligence is being applied to most tasks. With a half-billion dollar spreadsheet, it's not going to take much of a rounding error to make that software engineer's salary look insignificant.
You draw a distinction between "professionals" and people who make spreadsheets. Why is that a good distinction? Shouldn't those people have some professional standards too? What are you doing if you don't have time to do a good job on something? Heck, why is it okay for you Excel programmers to not have the same set of standards that the rest of us follow?
Following all these rules of procedure and specs and change requests isn't any more fun from the other side of the table. The point of that stuff is, if the product does not conform substantially to those specifications, the person coding it is (usually) liable. Reimagine your spreadsheet-writing life with the idea that you could be sued into oblivion for any losses incurred by your code, and you might develop an all-new appreciation for testing and specifications.
But you know, maybe it's your money on those spreadsheets.
Can you point out a single case where a programmer was held personally liable for losses incurred by their code? Just one?
Libility is usually limited to willful wrongdoing and negligence, and when it involves work done by an employee, it's the employer not the employee that's held liable.
But you're right about the financial liability: I've never heard it happen; but what with Murphy's Law I'm still retaining a Liability insurance just in case it should ever happen to me.