The compound 不起 is not just limited to this particular construction. It's a bit like a suffix that succeeds a verb, meaning 'not within one's power' or 'can't afford to due to the possibility of dire consequences'.
Some examples of compounds that use 不起 include:
- 吃不起 (Can't afford to eat - either due to financial constraint or social constraint)
- 住不起 (Can't afford to live in - in a similar vein to the previous example)
- 受不起 (Dare not accept - probably in fear of its consequences)
- 伤不起 (Can't afford to suffer injury).
对 here means to face or to confront. So combining the two, you have 对不起, which literally means 'not having the capacity to face or confront'. The verb usage of 对不起 can also be explained in a similar fashion, which can be translated into 'to wrong' or 'to let down'. 对得起 would be a perfect antonym for the verb definition.
Reference: See definition 6 here.