Both “plaît” and “tait” sound equal.

But “plaire” comes from “plaisir”.

We can see there was a “il plaist” form in the past: http://www.cnrtl.fr/etymologie/plaire

Therefore, some vowels + “s” are reduced to a “î” or “ô” in modern form, like “hospital” => “hôpital”, “plaist” => “plaît” etc.