Why are there payment thresholds?

Pretty much all affiliate and partner programs require you to reach a certain threshold before they pay you. Since these payments are usually carried out automatically, it's probably not the manual effort that makes them do this.

I always assumed they use this to stay solvent and to minimize fees.
But is this all or are there more reasons for why payment thresholds exist?

Interest maybe if that is even a thing now days, I know it isn't any more where I live, lol? Or otherwise using the money somehow to their advantage in the interim.

Payment processing is an expensive industry and as we all know, Paypal and banks charge quite a bit.
So, sending 5 bucks to thousands of affiliates can get really expensive for the network which is the reason they keep a threshold.

The second reason is to identify they only get serious affiliates who are confident of bringing them a good number of sales and not newbies who would give up after making their first five bucks.

Lastly, this also allows them to know the quality of the leads, a lot of people use shady tactics which might get missed out in one or two leads but if you keep repeating it again and again, it would become easy for the network to identify and ban your account.

One reason is that payout methods usually have fees attached to them. Some more, some less. The fee of a wire transfer is around 50 bucks, which is payable by the recipient, so unless you have at least $50 accumulated in your account, networks are unable to pay you. This is an obvious example as to why thresholds exist.

Its mainly to keep tab on quality of traffic and affiliates.

Don't forget all the people who get close to the threshold then stop for one reason or another.

1000 affiliates get $90 then stop, they have $90k to keep.