But 100k isn't a big salary, if you live in London or the SE, where most jobs paying those salaries will be located.
You're taking home just over £5500 a month from that salary after tax, but you'll also be paying into a pension, so it will be less than that.
Average rent for a two bed flat in London is about £2500, before council tax and bills. So that's well over half your salary gone on rent and bills. If you've got a child in nursery, that's another £1000 at the bare minimum. Leaving you with about £1000 or less a month to buy all food and essentials, pay towards any student debt or other debt, and put some money by.
If you've got two earners on £100k in a household in London, or you're earning £100k in a part of the country where rent is £1000 per month, you should be having a very good quality of life, sure.
But if you're the sole earner for a family in London on £100k, you're going to be struggling to put anything by month to month.
And when you consider that many jobs that pay £100k+ do so because they require very long hours and a lot of stress, it's no surprise that people on that salary might feel a bit aggrieved that their hard work and stress doesn't get them a particularly luxurious lifestyle in some parts of the country - contrary to other people's expectations.
How wealthy you feel is all relative and while I can appreciate it might feel galling to see people on considerably more money than you complaining that their salaries don't go very far, it doesn't make it any less true for those people.
And for those saying 'oh on your 100k salaries paying for private school etc' - I work in a private school and the fees are so high after VAT that the school fees alone are £4k per month now. And that doesn't include lunches, school bus, trips, etc. I can assure you, no one on a £100k salary with a mortgage is able to pay for school fees as a sole earner.