Belarus seems like a safe "friend" to Russia for the near future, at least, so I don't see the need for Russia to annex them.
Putin needs leaders of other countries to support him, or at least to "stand by his side" during photo opportunities. It seems the only images/video that we see with Putin, where a large group of other leaders are standing with him, are those from past USSR satellite republics.
Having puppet states is better then annexing a country, there is too much opposition to annexing a country. The Warsaw Pact was an alliance of Soviet puppet states, it was better for the Soviet Union to have puppet states such as East Germany and Poland, instead of annexing them.
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