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Because bigest would make no etymological sense?

digest

late 14c., in reference to Justinian's law codes in ancient Rome, from Late Latin digesta, from neuter plural of Latin digestus, literally "digested thing," noun use of past participle of digerere "to separate, divide, arrange," etymologically "to carry apart," from dis- "apart" (see dis-) + gerere "to carry" (see gest).

ingest

past participle of ingerere "to throw in, pour in, heap upon," from in- "into" (from PIE root *en "in") + gerere "to carry" (see gest).

-gest

"famous deed, exploit," more commonly "story of great deeds, tale of adventure," c. 1300, from Old French geste, jeste "action, exploit, romance, history" (of celebrated people or actions), from Medieval Latin gesta "actions, exploits, deeds, achievements," noun use of neuter plural of Latin gestus, past participle of gerere "to carry on, wage, perform," which de Vaan says is considered to be from the same root as agere "to set in motion, drive forward, do, perform" (from PIE root *ag- "to drive, draw out or forth, move").

bi-

word-forming element meaning "two, having two, twice, double, doubly, twofold, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, dis-, Old English twi-, German zwei- "twice, double"), from PIE root *dwo- "two."

https://www.etymonline.com/word/bi-#etymonline_v_11111