Because if there was only a single source (for example if the other one was out-competed), they'd have to pay 30% of their revenue for the privilege of being in the FabStore.

Samsung has already announced that their frontier node (what they call 1.4nm) is going to delayed at least two years, and issuing statements calling it into question at all. Intel has announced that they will only do what they call 14A if they can get a partner who will promise to use it in significant volume.

As of this moment, the only company that is definitely going ahead with that next generation node is TSMC. The other two companies capable of doing so are both signalling that they will only do it if they get a partner who promises to use them for significant volume, not just as negotiating leverage against TSMC.

They always are the first ones to use the most advanced node by TSMC, the designs probably are only compatible with that particular process. Have not heard of apple using samsung for SoCs.