Let´s try mining on land first...
Or in Tesla terms we could say
"Let's allow someone else to make our own batteries and suck up whatever quality/technology they want to sell us"
"Let's make cars with a high human component because that is how it has been done"
"Let's use Nvidia computers and just suck up the power drain and heat loss"
"Let's make new factories the same way everyone else does and take 2-4 years to come to production".
Tesla is about firsts. Polymetallic nodules contain at least 4 of the minerals Tesla requires and in sufficient quantity for their needs. On land Tesla would need to create contracts with multiple different suppliers and the minerals, in the volume needed, are mostly in places in the world which carry issues with reputational damage for buying them.
Tesla is all about firsts and that is what got them where they are today. If they start going for seconds, now, they may just stumble at the final hurdle.
If they do this, nobody can just come in and offer the miners a better deal or lean on the miners. Tesla would BE the miners.
There is enough cobalt, manganese, nickel, copper and iron down there to unlock every Tesla vehicle for the next 100 years. It also has viable quantities of aluminium.
There isn't a company in the world, today, who has more to gain by mining this resource. Or more to lose, potentially, by not.