In part, it’s what @alahmnat said - pretending to be democratic is what dictators do to fit in these days. And it does work - it’s always “president Putin”, while for example Lukashenka of Belarus, who is somewhat less subtle about the “for life” aspect of his presidency, gets branded “Europe’s last dictator”.
There’s also internal considerations. Even though everyone knows that Putin’s approval ratings and vote counts are bullshit, nobody knows by how much. And when the populace has only one 90+% figure as reference, there’s bound to be some psychological anchoring - “sure, he’s not that popular, but there still must be plenty of people who like him, so no use grumbling”.
Plus, fake elections work as a sort of character assassination of the whole concept of democracy: when you’ve learned from experience that elections are a charade where the result is always decided in advance, it’s hard to be excited about a solution that involves more voting - except done properly this time, unlike the last time when fake elections with one candidate were replaced by fake elections with multiple candidates.