It was built that way. But the Raiders held out, and in the meantime the 49'ers managed to sell all the luxury boxes, which gives them most of the income from the stadium, leaving the Raiders with only gate prices (which are shared with the rest of the league). So all the financial incentives for the Raiders to move their are gone, as the 49'ers have all the non-shared income.
If the Raiders had agreed at the onset, they could have split the luxury boxes, but that ship has sailed.
For the same reasons, there's no financial incentive for the Raiders or Chargers to move into a Rams owned stadium, so neither team will be moving to LA.
With the Rams actually owning the stadium, there is no way they're letting either team have some of their non-shared revenue.