Because ㅅ irregular is not elimination of ㅅ but changing into 으 There was ㅿ character in ancient times. The pronunciation of ㅿ is similar to /z/ It was eliminated in the first place nowadays. It was changed into ㅅ in the last place nowadays. 짓다(build) was 지ᇫ다 in ancient Korean. 낫다(recover) was 나ᇫ다 in ancient Korean. The rule is that ㅅ changes into ㅿ(light ㅅ) and then ㅇ(null sound) 짓다 : 지ᇫ(다) + ㄹ/을 것이다 -> 지ᅀᅳᆯ 것이다. -> 지을 것이다. 낫다 : 나ᇫ(다) + ㄹ/을 것이다 -> 나ᅀᅳᆯ 것이다. -> 나을 것이다. c.f. 사(다)(buy) + ㄹ/을 것이다 -> 살 것이다.(pronounced as short vowel: [살 것이다]) 살(다)(live) + ㄹ/을 것이다 -> 살 것이다.(pronounced as long vowel: [사알 것이다]) 지(다)(lose) + ㄹ/을 것이다 -> 질 것이다.

Hello ^^ 집을 짓다 (normal) i build the house 내가 지을 집.(future) the house which I will build 집을 지을 것이다.(future) () ganna build something that is house 집을 질 것이다.(wrong) well, simply, cuz 질 is not a correct one. maybe you heard about 질 집 but it is just shortened sound of 지을. sound 지을 as fast as you can. maybe you will understand.^^ 지을 것이다- 짓다+~을 것이다. as you know, ~을 것이다. gives a 'future' tense 질-- a short form of `-지를' for example 그녀는 웃질 않는다. -> 그녀는 웃지를 않는다. thanks