Large corporations have an effective tax rate below the nominal one due to antics with exceptions — while smaller businesses get hit harder. What’s called “progressive” often ends up regressive due to the imbalance in ability to do finance trickery. Rich people will always find or create exploits in the law, so those “high taxes on mega corps and billionaires!” hit the middle class and medium sized businesses: they’re who bear the brunt of taxes, in the US.
You get out of doing that unnecessary damage by removing the game: no whatever exceptions, just a flat rate everyone pays.
What’s called “regressive” is actually progressive in that it lightens the tax burden on the middle class and medium businesses, while making the richest pay a fair share. (Also, simpler codes are harder to cheat.)
Discussing the tax burden of the poorest is a misdirection used by politicians: the poorest in the US receive more benefits than they pay, so it’s a question of how that burden is split between the middle class and the upper class.