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IIRC they need to fly the cold war jets like this - below radar - as their electronic capabilities are too bad, so a more modern russian jet could launch a missile at them just before the old Ukrainian jet sees a threat.
That is one of the reasons Ukraine urgently needs more modern material like enough numbers of F-16. While also being not the most modern jets those at least have some decent radar capabilities and can also be used as interceptors for air defense.
They don't have look down shoot down radar.
Being low gives them cover from ground clutter, they can try to notch doppler radar, coupled with horizoning Sams like buks and s300s.
Always better to be low unless you're shooting someone, then you fire high to give it much more range.