Depends on if it was his lawyer's strategy or if it was his own idea (and I'm guessing it was the latter, just based on the prior behavior of "can't shut the fuck up and stop digging own grave")

If it was his idea, it often goes like:

(a) I am very smart, (b) I've always gotten my way by being very smart, (c) I (think I) am smarter than everyone who will be in the courtroom, and therefore (d) I'll get my way out of this by being smart.

Lot of smart people in fields other than law think that it'll translate to law. It doesn't, typically.

You see a less extreme form of this on HN frequently where people try to treat law as some boolean code thing, with statements like "well technically it says XYZ and because of ABC constraint...", when the reality is that any judge will take a look at the situation and wave the heavy wand of "spirit of the law" or whatever.

If it was his lawyer's idea, then I guess probably he decided case was already 99.9% lost, might as well hailmary it and see what happens.

In short: His goose was already cooked, he did so much brazenly stupid stuff in full view of the public and ran his mouth off after doing it that the case looked like a slam dunk. If he went on the stand he at least had a chance of convincing the jury that he was unaware that everyone around him was committing fraud. He didn't have much to lose.

Obviously nobody knows for sure but him and his lawyer, but almost definitely he just didn't listen to his lawyer because he thought he could convince the jury.