Abu Hurayrah quotes the Prophet (SAW) as saying:

The one who did not fast on a (particular) day of Ramadan without a (valid) excuse will not be able to make it up by fasting a (whole) lifetime, even if he were to do that (Ibn Majah, Sawm: 14)

As it is understood from the glorious Hadith, by breaking the fast without an excuse, one makes a mistake which has a huge responsibility and so difficult to redeem.

Although one can compensate by fasting the Kaffarah Sawm of 60 days for breaking his/her fasting without any valid excuse, He/she cannot get the reward of that broken fast by any means. He/she misses out that grace.

As our Prophet stated, he/she cannot regain the same reward even  fasting yearlong rather than a Kaffarah Sawm.

Because it is impossible to regain the reward, grace of a timely worship by making Qadha of it.