Why is it that android apps aren't reviewed before entering the play store, unlike ios apps that are reviewed a bit too much?

They are been reviewed by android also but ios has strict rules so that the app should be bound to that.

Sooner android is also making such rules to make more efficient like Ios.

Because simply Android is "Open Source", that leave the opportunity for anyone to publish their app. Afterwards, the app users could review it, not developer, nor Google's personnel. As for iPhone, each and every app published in their app Store must be verified and tested and may be even modified by Apple team accordingly. Therefore, they may leave their review.

They are used on private/individual gadgets which does not impact much on network setups.

In iOS, Apple dost not allow apps on app store directly due to security and other threats.It checks that app is code sign by trusted app developer account and other things before allowing app to go live in app store..

In android,this is not done..

Because Google gives freedom to the app makers to makes application which can be used easily by the user and any one can put their apps in playstore and it is not possible to certify each and every apps in play store because they too many in numbers.

Google acts as an free platform unlike IOS which is restricted. Moreover Android does review the Apps but has a less filtering system compare to IOS 

Apple has more stringent rules for apps unlike google. I think this is better, it's far more ethical and secure than google's system.