Because of course it's a scam when you step back and see how all the pieces fit together. But when the victim is in the middle, they're being confused in a lot of ways:
* The scammer keeps the focus on the upsides (job, office supplies) which are good but not unrealistic.
* The scammer is keeping the victim overwhelmed with interviews, paperwork, training, etc. The actual scam part is a small percentage of the victim's focus. This is classic misdirection.
* The scammer gets the victim to commit a lot of time and energy. Loss aversion and social norms stops people calling out slightly suspicious activity immediately.
* The big one is that the scammer drip feeds pieces so that the victim doesn't see the big picture. It's not like the scammer lays out the scheme on day one. They start with the company providing supplies (not suspicious), bureaucratic fumbling meaning the process changes (understandable), the new hire submitting the order themselves (ok), etc. Each step is logical and only slightly peculiar in isolation. Once the victim is comfortable with each step they are fed the next lie.
The last one works because even if people are suspicious of new information, once they've decided something is true they rarely reconsider their position. In this case the scammer got the victim to believe that the job was real, and the scammer was a legitimate HR person before introducing the scam element. If you believe the company is real, why would you imagine the check is fake? If you truly believe the check is real, how does the transfer risk your own money? They build a foundation of trust by layering a lot of little lies.