OEM parts have the reputation to be the most suited parts for the vehicle. There are important scenarios where somebody wants to use the most reputable parts instead of alternatives.
Think In-house maintenance for fleet vehicles:
There is not only the maintenance work but some also needs to comply with the (often informal) company culture. Should there be rumors that Person X is endangering employees/drivers lives by using cheap brake parts the accused one has a hard time producing evidence that the non OEM parts are suited for the job. Needless to say that such rumors are a career killer, especially after something happened and it involved high speed.
While there are norms and codes on what a brake part is supposed to do (Example: The European norm "ECE R 90" in a very rough sketch allows pad performance to deviate up to 15% from the OEM part performance) somebody first needs to explain that and also must admit that the parts could be in fact not as good as an OEM ones.
The best way to get out unscathed is to produce written evidence that only OEM parts got used, so only the best (and priciest) parts where used.
The OEM producers know about this and adjust their prices accordingly