More to the point, heads of state do not normally kowtow to one another, unless they're the clueless Barack Obama bowing to the Saudi king. Elizabeth II is the symbolic head of state of the U.K. and associated Commonwealth dominions, Donald Trump is the symbolic head of state of the U.S.
----------
Many nations have a symbolic head of state 'president' or royal who is basically an apolitical figurehead, and another very political person (often a prime minister) who is the executive head of government.
In the U.S. we combine the symbolic and executive functions in our president which is why he both pardons a turkey every Thanksgiving and has legislative veto power, officiates at the Easter egg roll on the White House lawn, and appoints Supreme Court justices, and is the government CEO.
The busy two-track schedule is a big part of why most presidents age (in appearance) 20 years during 8 years in office. They are "on" nearly all the time, and it's wearing.
It will be interesting to see if the office ages Trump as it has most presidents. It is possible he has operated at this frenetic pace ever since he left school, and is accustomed to it whereas, for example, senators and governors often are not.
