The four ages are not simply laid end to end. Each yuga is given a twilight before it and a twilight after it, sandhya and sandhyamsa, and in the standard Puranic reckoning each of these is one tenth of the age's own length, with the two together making up one fifth of the total. The figure of 1,728,000 years therefore already contains its twilights: 1,440,000 years of the age proper, with 144,000 before and 144,000 after.
The same proportion applies down the series, so that every yuga is nine tenths body and one tenth twilight at each end, and the ratio 4:3:2:1 holds for the whole periods as well as for their cores. It is a tidy scheme and it is stated as such in the Vishnu Purana and the other sources that give the durations.
The twilights matter to how the ages are described. Decline is presented as gradual rather than abrupt, and the transitions are where the character of one age gives way to the next, which is why the texts can place events at the junction of two ages rather than inside either. The end of the Dvapara and the beginning of the Kali are handled in exactly this way in the traditional accounts of the death of Krishna.
A note on the number itself. Every figure in the scheme is a multiple of 432,000, and 432,000 is 1,200 times 360, so the whole system is generated from the 360-day nominal year of Indian calendrical arithmetic and the ratio 4:3:2:1. Whether that arithmetic was chosen for its properties or arrived at by some other route is not stated in the sources, and reconstructions of its origin are proposals rather than findings.