Because chances are, the costs of dealing with the customer are higher than their MRR. That means that every month the customer stays you are loosing money.
If you operate fully bootstrapped – this is a huge red flag. But even when you’ve got basically unlimited funds at your hands to grow at all costs you should not keep these kinds of customers.
Because even then your human resources are limited and you create high opportunity costs. So while your customer support and customer success managers are busy dealing with a bad-fit customer they could spend the time with a highly profitable one instead. A customer who does not only contribute positive margins but comes with sufficient upselling potential.
And besides the financial issue, dealing with that kind of customer is also a morale killer. If you consider yourself a strong CS leader or work for some, the customer should get fired.