Thinking about it with a toy model:
Assume you on average contact 25 people a day when you go out, and each of your housemates each also contacts 25 people a day. Also assume if they become ill, you're 100% likely to get it from them. (These are not necessarily realistic assumptions, so please don't infer anything from these particular numbers!)
This means that if you go out, you're effectively contacting 100 people a day (75 of these by proxy.) If you stay home you're reducing your total number of effective contacts from 100 to 75, so a 25% reduction. This is still an overall reduction in your total exposure.
Whether that reduction will be enough for you for it be worthwhile depends a lot on the specific numbers. These numbers include the overall risk of infection per contact (at present likely low but could increase, also depends a lot on where you live), how many contacts you and your housemates actually have, and the probability of getting it from them if they do get infected (probably a lot less than the 100% assumed here).