For men, is not praying in jamaat sinful?

No, you're not sinful, but why would you not do this when you know the reward is 27 times more, as the Prophet said in Sahih Bukhari, than praying individually. So always try and pray in congregation. Remember that every step you take towards the masjid is a good deed as well. And the angels make dua for you when you're waiting in Salah and after the Salah as well. So always try and pray in congregation.