Because exchange core called core for a reason. You can't let someone who you can't reach to code most critical part of your whole business. The core is the only thing that have direct access to your user ballance sheet, the subsystem that finalizes trades. You really want to keep the person in charge of core as close to you as you possibly can and ideally have at least 2 people working on it. Make no mistake - amateur can't do code review behind those people, and if they smart enough to build a core, they are smart enough to come back in a year and drain your wallets empty.