It's part of the game that needs looking at, and VAR could easily do it, it's not hard to see when players are on the blag and if contact is enough to make a player go down, they should also review them through the week after the events and dish out game bans if they are deemed to have cheated.
Mane made it easy to spot he was on the blag, he actually took a step after the contact as his instinct to cheat kicked into gear and told him I should try a dive here, someone has just touched my arse.
It's still subjective though mate, there can't be a hard and fast rule with penalties. It's still going to boil down to a matter of opinion in any scenario. Some you'll agree with and some you won't. Them's the facts and we have to accept it.
Scenario: You're a striker for Everton in the last game of the season and the stakes are high (the team needs a goal to avoid relegation / win the title / qualify for europe - delete as appropriate).
It's the last minute and the ball falls to you in the penalty area. The goal is at your mercy and, just as you're about to strike the ball, you feel
a firm barge in your back / kick to your standing foot that
isn't sufficient to knock you over (you could definitely stay on your feet)
but it is absolutely enough to knock your balance so that you
can't cleanly strike the ball, what do you do?
Do you:
A) Try to strike the ball and potentially miss in a crucial game.
B) Exaggerate the contact, give the ref a decision to make and get the penalty.
Now if you pick A, fair play to you but there are many in that situation that would choose B. The pressure is so high in the modern game that a big miss could be catastrophic and there have been multiple instances where fans have literally berated their own players for choosing to stay honest and stay on their feet when they could've gone down.
It's been rife in Football for decades now and it happens with all teams all over the pitch, not just in the penalty area. Until the system changes, and I don't imagine it ever will, you'll see it happening week in week out.