you can buy Bitcoin Mining Hardware tools throw paypal but why cant buy bitcoin by paypal
is this logical

?
From PayPal's perspective, it is logical, buying a physical item is different from using their service to operate as a currency exchange which is against their terms.
You may not use the PayPal service for activities that involve currency exchanges or check cashing businesses.
So according to this policy, selling of Bitcoin comes under currency exchange business.
https://www.paypal.com/us/webapps/mpp/ua/acceptableuse-full#prohibited-activities