They can't afford not to. If they don't put something aside while working then when they get to retirement they will be left with just the state pension.
On minimum wage of £11.44 from April a 37.5 hr week job will pay £22,369. Without pension that would give a take-home pay of £19,625. With pension it would be £18,980 but you'd have £1,290 in a pension. That £1,290 only costs you £645. 50 years of no pension would give you £32k which I assume would be spent and not saved so would give no return but contributing to a pension and assuming a modest 4% growth above inflation would give you a pension pot of £206k. A £206k pension pot can get you a tax free lump sum of £50k and a yearly income of £10k.
If you take the £645 and invest it independently and not in a pension (with the same growth) you will have £103k. The pension is the better option.
Pensions are not a scam and the earlier you start the better.