They are not, the fact the Russians have lost so many planes is a testament to their inability to suppress the Ukrainian air defense.
Fighters are capable of flying far higher than MANPAD ceilings the only reason to fly that low is to employ visually guided weapons under cloud cover or to attempt to hide behind the terrain against longer range air defense systems.
Both of these issues can be resolved. By suppressing the enemy long range air defense and by employing laser guided weapons along with ground based designators.
Also fighters can evade missiles, google f16 dodges 6 sams for an extreme example.