True they use the api library provided by the providers but they do everything Blesta needs and callbacks themselves so they know it works fully and it would also be harder to use a third-party library like omnipay, and try and get every payment gateway to tell Blesta what's it's doing. But if people do want omnipay you did a module for it But I'm bias as a few know and try and keep third party modules to ones I need and know they maintain it and donate if needed or pay a monthly or one-time price.
Eg: if I sold vps: SolusVM extended would be the only one I'd need. If I sold cPanel shared hosting I have a choice, free cPanel Extended, the paid one or (maybe) ModulesGarden's one if they decided to make their own one again.