Governance and diplomacy is not a short term venture. Trump will not be president forever and you cannot undo the act of siding against the US in favor of China.
There are a lot of decision-making processes in governance that involves a lot more than 'we don't like the current president'.
Interests can be short-term. US having no problem siding with China still takes into account the long-term interests. Generally Western world members do the same, even if that goes against the short-term interests.
There are a lot of decision-making processes in governance that involves a lot more than 'we don't like the current president'.