Because it's 1920 solid wall and it wasn't usually installed then. This is the only wall where ground level is at or above the floor level.
It may well be true, but building "regs" as was, where followed, even then required some form of dpc.
Our house is 1920s rendered brick, and everywhere has several layers of slate dpc embedded in lime mortar.
As we all know, slate doesn't make a continuous damp proof layer, but it does slow the moisture down.
A lot of people with non visible slate dpcs are under the impression it's either missing or failed, but in reality is as adequate as it ever was.
Usually it's changes such as the raised ground levels implicated here that can be resolved to fix the issue, despite the fact that the dpc was never perfect even where it did exist.
Injection damp proof course can certainly help the performance of slate dpcs or other imperfect ones, but other less invasive solutions can be followed to bring things back in balance.
Not knowing the exact op situation I can't say how much applies to them, but it's hard to be certain about 100 year old construction without investigation!