Because firstly it is much more expensive (1 school logo polo top for £7.50 when I can get two plain ones for £5).
Secondly the list states "only shirts (l/s or s/s) with ties. NO open neck blouses for girls. Dd2 hates tight things near her neck or feeling restricted thread hated the shirt tie combo even with top button undone so I have got her blouses with an open neck as she will be comfier.
Db was horrified (his daughter is also starting school at the same time) and he said I am wrong to deliberately get the 'wrong' uniform