To use the example Mhairi Black gave. If you're down in Westminster, and you're scheduled to vote at 11pm. But you've done all your work by around 7pm.
The temptation is to go and kill time with a pint, rather than sit around in the office twiddling your thumbs. And one pint so very easily can become a second pint, and then spiral to too many pints.

It's especially more tempting when your drinks are heavily subsided.

So why on Earth MPs are allowed to cast votes on matters that impact over 60 million people while hammered is absurd.

Spencer was right on the money, as was Skinner. In fact my one regret is that she didn't go far enough.
Mhairi Black for example since stepping down as an MP, has spoken about how the culture at Westminster effectively encourages drinking.

As Skinner put it, when he was a miner and working down in the coal pits, if someone came down stinking of ale then they wouldn't be there long. They would be removed, sent home, and if it was a first offence then docked a day's pay.

That profession isn't alone. I can't think of many where you would be allowed to stagger around, and carry on as normal.
In fact in some you would be considered such a liability, that you would face suspension, termination, and possibly criminal negligence charges.