Its not cheap to produce, warehouse, and distribute lots of parts. A £5 part may cost ~£1 in storage costs a year.
The big problem with car spares is the assemblies they come in as manufacturers don't expect "mechanics" to have the skills to fix things these days, that and the fact they don't have to keep as many parts in expensive warehouses.
Its the assemblies that are expensive when a fix by someone that can do it might be pence.