Opting out pension scheme when you are a postdoc abroad
But still, can someone tell me a reason to not opt out the pension scheme?


Because you are likely to get more out of it (at old age) than you pay into it (now) — and even if you don't, it may be safer for you to put away money in a way you cannot (easily) access until you no longer have direct income from work.

Both you and your employer are opting into the pension scheme.  Despite recent threats, it is still a defined benefit scheme.  If you and your employer have paid into the pension scheme for x years, you will get money out of it after you reach the retirement age, regardless of where you live.

Depending on where your next (long term) job is, your (next) pension scheme may have agreements which might allow you to transfer the money paid into your UK pension scheme to your next pension scheme.  Consult a professional.

I have pension savings from temporary (academic) positions in Sweden, Canada, the UK, and Germany.  Once I'm fairly certain to remain in the same country until retirement, I will consult with a professional for advise on whether it is possible and wise to pool them all together into the fund of the country where I will then live.