n.b. This argument is ~exactly the same for donating your time to charities. The current company which you sell your services to has a way to metabolize them and will pay you (presumptively high) market wages for doing so; a charity chosen randomly by you at a time convenient to you may have no infrastructure to metabolize your services nor a charity-perceptible need for e.g. web development.
If you're feeling charitable you can virtually always find a charity amenable to you which has outcomes they'd like to cause in the world but for lack of money and help them vis the lack of money.
If you're feeling charitable you can virtually always find a charity amenable to you which has outcomes they'd like to cause in the world but for lack of money and help them vis the lack of money.