You can't just give everybody who works full time a minimum of 20k or whatever just to make sure it's more than benefits.
Not every company has CEOs at the top it can be blamed on because they have a big wage, plenty of small businesses would be up shit creek if they suddenly had to pay their unskilled workers £20k or whatever rather than the £13 - £14k on minimum wage. That's £7k per person they have to pull from somewhere every year, they will have to whack their prices up to make that up.
What if there is a small design agency for example looking to hire a graduate fresh out of uni, usually they would be able to give them £15k - £17k starting out. They don't have much experience but they have qualifications. That suddenly jumps up to £23k - £24k because it has to be more than benefits.
To cover the extra wage everybody had to pay people who worked for them prices and services would go up. It would get to the point when products and services are too expensive for people on benefits, what happens then? Benefits go up to match because then people can't afford to live because everything is too expensive. The multiple people involved in baking, transporting and selling the loaf of bread or pint of milk would all be getting pay rises, who covers it to prevent everything shooting up?
Yes, with big massive companies the CEO's could lose a few grand to cover it. and have 60k instead of 120k. What about small businesses with 5 - 10 employees where they would have to give them all thousands more a year just to make sure it's more than benefits.