Because he has no legal defense. He uses public political defenses, because he it's all he has. And it seems to be at least moderately effective. Ex: He's never going to be removed by the Senate.
It does appear that way. Delay, deny, destroy, seems to be the defense. The longer it's delayed, the more time they have to build their phony political narrative.
But I think once the real accusations are laid out, it may not be nearly as effective as it appears to us...since really we're just waiting for a year+, no real fighting back. So far, it's been Trump/Fox/Fake News engaged in the fight, with no response from DOJ. it's been entirely one-sided. The media has ridiculed and poked holes in the right wing narrative, sure, but that's like the crowd heckling them. They are in the ring, shadow boxing and the fans are cheering.
Here is the fight:
1. The Public Case: Do we have enough evidence for Trump's base to oppose him on ethical, legal, moral, grounds - Democrats would do this quickly. Republicans have decided to support Trump on this for now.
2. Criminal Case #1 Mueller fingers Trump's campaign and/or Trump for conspiracy, and related charges like perjury, obstruction
3. Other civil/Criminal cases (Trump foundation, possible tax evasion, etc)
4. Democratic leadership in the House uncovers numerous ethical and criminal violations related to the enormous breadth of this debacle, possibly leading who knows what for Trump campaign folks, Trump, and others.
Clearly #1 isn't effective, we can't rely on Republican voters, Republican House, or Republican Senate to be good stewards of our nation.
But we haven't seen #2 yet, it's just a tightening noose.
And #3 and #4 are in the wings, but both seem promising, especially #4.
I think the combined pressure of Mueller's case wrapping up, and House hammering, if there is sufficient evidence of crimes legally presented against Trump (not indicted, just presented), obstruction, perjury, and conspiracy, they may buckle.
Point being, the Senate has only been tested on "it looks bad for Trump". They have not yet been tested on "criminal evidence on multiple felony charges has been irrefutably laid out by Mueller/others". I believe with sufficient evidence from DOJ they will do the right thing, if impeachment passes.
