Trump lies because it works for him - here and now. It suits his purpose and his purpose is usally to make himself look good, because he is anarcissist.
FTFY. I can recommend the article that Bob001 linked to in the thread about Trump's dangerous mental illness:
And another by a shrink:
https://medium.com/@vgwcct/a-duty-t...g-the-diagnosis-of-the-president-371354142a02
It explains
1) what makes Trump a psychopath instead of a narcissist. It also explains why the difference is important.
2) how he lies without considering the long-term (sometimes just a couple of minutes) effect of his lies. It doesn't really make him look good to lie about things that are easily revealed to be lies: the number of people at his inauguration, the availability of coronavirus tests, the allegede beneficial effects of hydroxychloroquine and bleach, having the virus under control. When his pandemic advisers have to correct the lies he spreads only minutes after he has told them, it becomes obvious to a majority of Americans that he is lying. Some of them - fewer and fewer, apparently - don't seem to mind, but the majority are made aware of the fact that he is a liar who can't even stop lying when his lies are easy to expose as such, and that doesn't really work for him.