Because the like control on a project, journal or publication is not tied to any competition schedule. It has no time limit, so it stays available after a competition's voting window has closed.
The like button belongs to the work, not the competition
Every project, journal and publication on UNI carries a like control, including work that was never entered in a competition. It is a general appreciation signal on that piece of work. Nothing in it switches off when a competition's voting window closes, which is why you can still use it.
To like a competition entry you have to open the work itself. Entry cards on the competition page do not carry a like control.
What is time limited
The Vote Now button on a competition page is time limited. It appears only while that competition's voting window is open, and it is removed once the window closes.
You will find the voting window under Schedule on the competition page. Competition dates are shown in UTC and labelled that way, so check the offset from your own time zone.
A like recorded after that window closes still registers on the work itself, but it does not count toward that competition's public vote tally or leaderboard placement. Only likes recorded up to the moment the voting window closed are scored, for competitions that set one.
Rating is a separate thing
Rating is not the same as liking. Rating scores a project against named criteria instead of giving it a single mark of support.
You can rate projects on a Semester, Annual, Studio or Institutional membership. Monthly and Weekly memberships do not include rating. You cannot rate your own project, and you can give only one rating per criterion on any given project.
See UNI Membership for what each plan includes.
How results are actually decided
Competition results come out of jury evaluation, and awards are assigned by the UNI team rather than generated automatically. See How jury evaluation and results work.
If you think voting has been manipulated
Report it, and include what you saw. See How can I report suspected fraudulent voting in a UNI competition?.