If we come back to the company choices whether they are willing to take an overhead cost for managing the data or outsourcing it. Do note that, KYC/AML management doesn't solely imply a simple thing like verifying whether the user is legitimate and matches the provided documents. There are many levels within this subject like fraud analysis and checking if one identity is not part of some blacklist or something. And it requires data whereas some companies who specialized in that kind of subject already have.
Let's check a third-party platform that Roobet use, Veriff and Seon. If you are going to their website, you can clearly see that they are solving many problems that are worth outsourcing if some companies are dealing with fraud or identity business requirements.
I understand that it takes time, but you could literally just hire one or two people for that job and be done with it and roobet is a multi-million dollar company that keeps on giving away a lot of stuff to the gamblers as well. Meaning they can definitely afford to hire some people, hell hire a dozen people if they want to, sure that would be useless cost but they can afford it at least it looks like that.
This is why I am not entirely sure the reason would be the hard work it would put. I feel like it is just to make sure that they separate the KYC to another company, for "just in case" reasons, it is adding a layer of security and that’s better.
A sector that is related to identity verification, fraud analysis, and money laundering detection can not be handled by one or two people. And yes Roobet might be able to afford the team to manage regarding the sector. But it came back to profit, what decision is the most efficient yet profitable for the company? Well, it's absolutely to use the third party, not to mention, the data they all already have for the analysis.
I believe there is no "just in case" reason. Running and choosing business decisions should mainly get the most profitable case as possible, so choosing a third party for identity and fraud management is actually predictable and favourable in business aspects, even not merely because of security reasons.