Why? The same reason he lies to everyone else. He is a self-serving egomaniac who purposely sews seeds of confusion, because he figures it serves his personal agenda. In fact, his avalanche of lies has effectively prevented much scrutiny being paid to any particular one lie. And that is how he escapes accountability. ...that and his penchant for preemptively accusing rivals and would be watchdogs of the very same ethical infractions he himself is actually guilty of. That way, his detractors look like sour grapes with their own integrity problems. He is such a snake of a con man!

They all lie....

Why does any politician with nothing to offer ever lie?

He thinks he can control people by telling lies.

HMIM, (His Mouth Is Moving)....this is the first lie he has told, or you just landed from Pluto?

Trump just figures that anyone dumb enough to sign up for military service will believe him.

Please note, that I meant Trump thinks its dumb to sign up for military service (which is why he paid a doctor to get him out of Vietnam). I myself do not think military service is dumb.

Although I do think teh stated reasons for some recent wars are pretty dumb... like "weaons of mass destruction" in Iraq, which they didn't actually have.

If he weren't able to lie, he would not be able to speak.

When was the last time he did not lied to anyone?

Because he can

For the same reason he lies to just about everyone - he's a mindless dolt, who clearly is mentally inferior to just about any inanimate object.