Why wasn't MH-17 flying at 33,000ft before it entered Ukrainian airspace?
This is normal.

A 777 on the AMS-KUL route cannot climb to FL330 right after takeoff - this would only be possible maybe 3-5 hours into the flight.

Airliners fly as high as the available thrust (and ATC) allows, because the thinner air allows to fly at a higher lift coefficient and less drag overall. The limit is given by the combination of available thrust and the actual mass of the aircraft, and it will climb higher during a trip as fuel is burnt off. The maximum range is possible at one particular lift coefficient, and to maintain this during a long trip means to increase altitude, so the lower density compensates the lower aircraft mass. Engine thrust is normally sized during design such that this optimum altitude can just be reached in normal operation.

On eastbound routes in controlled airspace, pilots are encouraged to fly at odd thousands of feet (depending on the local rules), so the climb will not be continuous, but stepwise. See this answer for more on separation rules.

For a rather lengthy answer with more background on the theory how to pick the ideal flight condition for maximum range, see this link.