because -stat is a Greek root. merriam-webster borrowed from French aérostat, probably back-formation from aérostatique "of aerostatics," with -stat (after

because -stat is a Greek root. merriam-webster borrowed from French aérostat, probably back-formation from aérostatique "of aerostatics," with -stat (after héliostat heliostat) taken as the Greek agentive element -statēs "one who causes to stand" — more at -stat