We do it for two reasons:
1. We don't think it makes any sense to use other periods. The bars have only the purpose to compress the market information in a smaller size. They don't change the market. The smaller the period is, the fewer details we loose in exchange for the bigger size (slower calculation). We don't see the benefit of making the software more complex only to provide intermediate periods.

2. The standard periods always work On the other hand, other periods may not work. For a reliable backtest we want to fit a round number of bars in one day. However, if we have a period of 61 minutes or 7 minutes we will not be able to do it.

I just calculated the possible periods in Minutes that can work if custom periods were possible:
1,2,3,4,5,6,8,9,10,12,15,16,18,20,24,30,32,36,40,45,48,60,72,80,90,96,120,144,160,180,240,288,360,480,720,1440