im not confident but i think its because the 수줍다is an adjective and 줍다 is a verb. They look so similar but have definitely different meaning! so they dont have any relationship each other even in grammatical part! Furthermore, i think, conjugation is sometimes very irregular between verb and adjective. They both end with -다 form and its just Korean's own feature. in conclusion, i think just getting used to it is the best answer:)

Only a PART of ㅂ ending verbs or adjectives are ㅂ irregular. 돕다, 굽다, 깁다, 눕다, 줍다 are ㅂ irregular verbs. 덥다, 맵다, 춥다, 사납다 are ㅂ irregular adjectives. 업다, 잡다, 뽑다 are regular verbs. 수줍다 is regular adjective. There were ㅂ and ㅸ in old Korean. ㅂ is similar to /b/ and ㅸ is similar to /v,w/ Some ㅂ of ㅂ ending verbs are ㅸ originally. ㅂ in 줍다 is ㅸ/v,w/ -> 오/우 ㅂ in 수줍다 is /b/ 줍(다) + 어요 > 주 + v,w + 어요 > 주워요 (ㅂ irregular) 돕(다) + 아요 > 도 + v,w + 어요 > 도와요 (ㅂ irregular) 업(다) + 어요 > 업어요(regular) 수줍다 + 어요 > 수줍어요 ( regular) NB. 곱다 as a meaning of "beautiful, pretty" is ㅂ irregular adjective. 옷 색깔이 참 고와요.The color of your clothes is so pretty. 곱다 as a meaning of "to numb" is regular verb. 추워서 손가락이 곱았다 My fingers have gone numb with cold.

difference between 수줍다 and 줍다 1. I am shy when people look at me. 사람들이 쳐다볼때 나는 수줍다. (adjective) as same expression, you can use '부끄럽다'. 나는 부끄럽다 (shy) 2. I pick up the book. 나는 책을 줍다. (verb) Usually, we speak .. 나는 책을 줍는다. (present) 나는 책을 주웠다 (past)