Yes, there's a reason - societal conditioning. Nothing more or less.
There absolutely nothing wrong with BFing a child of this age - none of the arguments against it stack up.
No nutritional value? Well that's bollocks, it's designed by nature not only to have nutritional value, but it's tailored specifically for your child. If the marketeers could genuinely replicate it, they'd be charging a fortune for it!
Only for the mother's benefit? You try telling that to BF school-age children. 😂 They'll quite clearly tell you you're wrong!
And any variety of objections that basically come down to people mistakenly thinking breasts are primarily for sex. No. Their primary purpose is BFing children. If they're also for sex, that's a bonus.
The UK has one of the lowest BFiing rates in the world. It is because of years of social conditioning that in our society we're conditioned to think allowing DC to BFing to 6 or 7 is somehow wrong. There is nothing objectively wrong with it, and a lot of benefits for both mother and child, including protecting against certain cancers, even.