Because EU law requires it, irrespective of the wishes of a national government or its voters.
The reason you don’t pay VAT in the UK on things such as food is for historical reasons, EU policy is to aim for VAT on all goods and services. More recent EU members such as Poland are forced to charge VAT on food although they are permitted to do it at a reduced rate.
If you are unhappy with paying VAT on bike lights I suggest get in touch with your MEP, perhaps they will be able to raise it at the European Parliament, that democratic institution which meets somewhere in Europe and rubber-stamps directives telling member states what they are allowed and not allowed to do. Good luck with that.