A lot of people are in lower income brackets and don't have the means nor understanding where and how to leave the country.

Another big factor is that many countries have essentially closed borders for Russian nationals - i.e. US is now deporting those who are fleeing - https://www.theguardian.com/us-news/2023/mar/18/biden-admini...

There are also few people who do want to stay in Russia and try to fight Putin there (like Navalny or Yashin).