Also, the vast majority of "executives" never see the inside of a cell, even pre-trial. They might be in a holding pen for a few hours, but they always make bail and if the judge/prosecution is particularly concerned, the suspect gets additional conditions such as some kind of monitoring/curfew. A great example is Paul Manafort.
From https://www.spectator.co.uk/2014/04/why-did-we-ever-spell-ja..., "The Guardian long persisted with gaol too but changed to jail in the 1980s, like the other English papers."
In Australia there's a move away from prison/jail/gaol and towards 'corrective/correctional/detention facility / centre' presumably to make the places sound more aligned with their political or social intent.
However each of the Australian state's active legislation still refers to them as gaols.
You're right about the high standard of proof required for tax cases, and the difficulties of prosecuting complex white collar frauds, but I'm not sure that we have to set the mens rea bat as high as we do. Plenty of other crimes can be prosecuted on "known or ought to have known" rather than having to prove intent.
That aside:
(1) We do jail executives, all the time.
(2) The reason maybe you think we don’t jail enough executives is because it’s hard to prove they did anything wrong. The criminal standard for proof is very high: beyond a reasonable doubt. When it comes to “blue collar” crimes, that’s pretty easy to meet. You wouldn’t believe the amount of evidence that exists in a typical drug crime! The defendant tried to sell drugs to an undercover cop; they found more drugs and an illegal firearm in the car; they searched his phones and got extensive evidence of drug transactions. (That is also why so many such crimes result in guilty pleas.)
But here the government didn’t win a case. It settled. You settle because there is risk you won’t win at trial. If you have to get accounting experts on the stand to argue about whether the thing that happened is illegal or not, good luck proving that beyond a reasonable doubt. (I don’t know what the standard is in France I don’t expect it’s much different.) Additionally, this is a tax case. It’s a “you put this number under section 3.3(b) instead of 3.3(c).” To secure a criminal conviction in a tax case, you need to prove beyond a reasonable doubt that the defendant’s interpretation of the law was not different from the government’s but that no reasonable person would have interpreted it the way the defendant did. Tax positions are carefully vetted and at a large organization like Google you’ll never take a tax position that isn’t at least plausible.
Stepping back, the problem is that you’re taking the government’s position at face value. At least in the US, the government prosecutes many cases that are, at best, based on aggressive or creative interpretations of the law. Of course the government says “Google defrauded the tax office.” Of course they have to say that. But dig into some high-profile white collar prosecutions. In the US they’re generally a matter of public record. They often are far from clear cut.