Sunk cost fallacy. You're exerting more effort with no reasonable expectation to see it repaid. The work you have already performed is a sunk cost, it's gone. The unpaid invoices are money you are owed but they apparently have no intention of paying you.

For all you know, they're not paying because they're bankrupt and dead in the water. That means not only will you not ever see the money they already owe you, but if you do any more work, it's effectively pro bono for a lost cause.

If you're perfectly rational, there is no psychological disadvantage to continuing to work for them but it's obvious to you why you shouldn't in the first place. Chances are, you don't act as rational as you think you do.

If a major client skips payments, it's perfectly understandable that you don't want to risk burning any bridges by insisting on being paid. But this is the fallacy. Whether the client is big or not doesn't matter if they're not going to pay you. Unless you want to build a business on not being paid.