Because either only Amazon sells the books I want to buy or the alternatives are other non-local shops that are usually far more expensive.

I live in Poland. I want to read books in the original English version. The main competitor to Amazon for ordering books in the original version would be libristo.eu, which is not Amazon, but it’s also not local and it’s far more expensive. On top of that, there are “local” online shops which engage in ordering things from Amazon to then only repackage them, as if they were bought locally. The books I’ve bought recently, all of them hardcover:

1. Lester W. Schmerr Jr., Sung-Jin Song, Ultrasonic Nondestructive Evaluation Systems. libristo.eu: 223.64EUR, Amazon: 83.84EUR.

2. Alan V. Oppenheim, Alan S. Willsky, Signals and Systems. Second Edition. Available from local Empik.com, but only in paperback version, and it’s a different edition. libristo.eu: 298.19EUR, Amazon: 237.81EUR.

3. Avinash C. Kak, Malcolm Slaney, Principles of Computerized Tomographic Imaging. Not available anywhere else than Amazon (to be fair, on Amazon I’ve also bought a used version, because there were none new).

4. David J. Griffiths, Introduction to Electrodynamics. libristo.eu: 67.11EUR, Amazon: 48.72EUR.

5. Thomas M. Cover, Joy A. Thomas, Elements of Information Theory. libristo.eu: 122.85EUR, Amazon: 83.38EUR.

As long as you buy hardcover versions, their quality tends to be a lot higher.

And I would not be able to find these books in any physical shop by just walking in. Even if some book was theoretically available, it would need to be imported.

(I’ll also preempt one possible criticism: it is not true that this state of affairs is caused by Amazon pushing out great local shops from the market. There used to be no easy options of getting technical books in original versions before Amazon, just translations, and only of a small number books in the most generic topics appealing to the lowest common denominator. You could maybe get Charles Dickens or Shakespeare in original version from Empik.com, maybe a Bruce Eckel book if you were lucky, but forget about getting a book like Elements of Information Theory. English proficiency in Poland is generally high, compared to Western Europe, but our local shops refuse to cater to it.)

For books in English, Amazon is considerably less expensive than anything local, and can also be significantly faster, instead of being a “special order”.