so why not stick with tabs which are more flexible, configurable etc?
Because indentation isn't just for code blocks. I can't stand method declarations with lots of parameters lumped together on one line with a single space between them; it's so much more readable to put one parameter per line. Calling these methods is a similar issue, it's unreadable when all the parameters are lumped together. Long expressions are another example.
Now to get everything aligned, you need to use a mixture of tabs (to begin the next line at the correct indent level) and spaces (to left-align the item). It's certainly do-able, but it's a break in your concentration. Then two weeks later someone else fixes a bug in your code and changes the spacing to tabs-only for their portion of the code, and the very next day someone else
Spaces remove this ambiguity, and it makes it easy for my less-meticulous co-developers to align things properly.