If Richard Nixon was smart enough to resign as president, then why wasn't he smart enough to tell the truth, about the watergate scandal?

He got himself caught up in the watergate scandal, he was told a lot about it, and then he lied and claimed, he knew nothing about it, and got away until tapes were released, that further proved he mew more then what he was telling. And that is what got the senate to start the impeachment proceedings, and when he was told that he did not have enough support for him to keep his job, without being pressured to he resigned from the his job as President of the United States. If he had been straight up honest, about what he knew, and told what he knew. He could have easily kept his job, the senate would not have had to get going with the impeachment proceedings, and he could have served all 4 of his last term as president. Without one having to turn over the presidency to a man, that was not elected to either president, or vice president.