You're asking for an in-universe explanation for something that doesn't exist in-universe.
For instance, Marc Alaimo plays several Trek characters. In-universe, nobody ever asks Gul Dukat if he's aware of his striking resemblance to a Romulan that Picard met once. In-universe, the two bear no resemblance to each other.
Out-of-universe, they could have used a different actor for each and every character in all five series (there is plenty of talent out there), but in most cases it's a better use of your time to ring up an actor whose work you've liked in the past.