Because London property prices are so high the first 'rung' on a housing ladder, i.e. a one bedroom flat, is often the most a couple will be able to afford in the long term.
Even for a well paid professional couple, the cost of moving from say a one bed flat at £450k to a two bed flat at £650k, would cost £20k in stamp duty alone.
Coupled with the fact that apartments in particular have been stagnant in most of London since about 2016, it's very difficult to justify the cost of moving once in.