Why did they need a team in Doctor Who: Time Heist?

In the Doctor Who season 8 episode Time Heist, the bank robbers frequently raise the question:

If [the Architect] can break in here and plant this thing, then why does he need our help?

This never seems to be answered properly, even when the Architect's identity is revealed. Towards the end we see clips of him planting the cases in various places; but if he can infiltrate the bank to do that, why not just do the whole operation on his own rather than recruiting the team as he did? OK, he needed the solar storm to be able to access the vaults, but why not get into the places he could infiltrate before the storm and then wait for the storm to hit before breaking into the vault?

I have a feeling this should all make sense clearly, but can't quite make everything match up in my head. I hope someone will be able to articulate a good explanation!