Why the potential at positive terminal is considered to be high?

I hope I understand your question correctly.

This is all caused by the fact that the "positive" and "negative" labels were assigned long before the electron was discovered.

Back in the early 1700s it was discovered that there were two kinds of static electricity, depending on the materials used to generate them. Initially it was assumed there were 2 different kinds of "electric fluids". Benjamin Franklin first proposed that the charges were caused by an excess or absence of a single fluid. He arbitrarily labelled the charge carrier as "positive". During the same period, batteries were developed and arbitrarily assigned positive and negative terminals.

Then, in 1897, J.J. Thomson discovered that electricity was carried by a "particle". He was found that this particle actually has a negative charge.

To this day, circuit analysis keeps using the old conventions and uses a hypothetical positive current - while keeping in mind that electrons actually flow the other way. In most cases this does not matter at all to the analysis.