Disposable nappy manufacturers spent lots of money on product development and launched several products to extend the time children are in nappies. Thus the 'pull up' was born and the number of children starting school in nappies 24/7 or using them at night increased, hugely. Look at the sale stats and sizing options.
This has been studied, fairly extensively, from a marketing perspective. Creating a product no-one actually needs and changing the 'stigma' around children being in nappies/pull ups beyond 2 - 3 years and normalising the use of nappies/pull ups at night has been a huge commercial success. Less so for the children, TA's and others who have to deal with this.
If you believe your child is not capable of using the toilet at 4 years old, using lines such as they're 'not ready yet' or you 'don't have the time' you've fallen hook, line and sinker for the nappy manufacturers' bunkum and are using products (such as pull ups and nappies with pretty nasty chemicals that prevent wetness) that are causing the problem.
It's not acceptable to send a child to primary school in nappies (disability/illness excepting of course). It's nonsense to do so in 2019 when there is so much help and advice and information about potty training and techniques available free, 24/7 online.
The whole phenomena is a manufacturing and marketing success, and a parenting fail.