Why do Finland and Estonia use the same melody for their national anthems, but with different lyrics?

The reason Finland and Estonia use the same melody for their national anthems is a result of shared cultural history, strong national movements, and the rapid, widespread popularity of the tune across both regions in the mid-19th century. It is also, actually, the tune for the anthem of the Livonians.

The melody was composed by Fredrik Pacius, a German-born composer and music lecturer who lived and worked in Helsinki, which was then the capital of the Grand Duchy of Finland, back then, an autonomous part of the Russian Empire.

https://www.nordicestonia.com/nordic/why-finland-and-estonia-have-the-same-anthem/#:~:text=The%20only%20difference%20between%20the,wrote%20a%20melody%20to%20it.