why mgt7 require CS signature for small companies

As per section 2(85) of the Companies Act 2013, small Company means the Company which is having paid up share capital of not more than 50 lakhs rupees or higher amount as may be prescribed and turnover of not more than 2 crore rupes or higher amount as may be prescribed.

as per section 203, read with Rule 8A, Every private company having paid up share capital of 5 crore or more shall have the whole time Company Secretary

But generally small company does not mandatory require to have company Secretary. So if there is no CS , then it shall be signed by Director but if Company is having CS , then it must be mandatorily signed by CS.