...and the fact that rules for testimony and cross-examination have been developed and challenged adversarially for hundreds of years; so if the really are as absurd as they think, they probably already would have been changed; and in any case, SBF's lawyers still have a chance to change them.
Here's why it seems reasonable to me. Imagine the counterfactual: If the prosecutor wants to ask him questions about that text without having him read it. If she reads him and asks him a question, he can just say "Is that what it says? I don't remember that." Then she has to hand it to him for him to read, point out where she read from, and give him a chance to read it, and then finally ask her question again. And if there is more than one section, or even more than one question on the same section, then the paper has to go back and forth.
If he reads it, then it establishes several things at once: The jury has heard the text that the question is going to be about, the defendant has read the original text and also has access to the context in order to answer questions, and the defendant has verified that what was read is what was written. It makes the whole trial go more smoothly, it's relevant to the trial, and it's not inherently insulting or humiliating (unlike cartwheels or farting on command), and doesn't fundamentally change the outcome of the testimony.
Maybe a real lawyer (or law historian) would have more to say; but in any case, defense lawyers have had hundreds of years to raise objections, and they haven't, so my "Baysean prior" is that there are probably very good reasons for the rule being the way it is.
ETA: Another advantage is that how the defendant reads the document, and their reaction to reading it, is part of the evidence the jury will need to weigh up when forming an opinion of the witness -- as the situation here described showed. And of course, remember that the same thing can be done by the defense to witnesses for the prosecution. It's not a tool that favors only one side or the other.