There are a lot of good reasons to not use AdSense on a site of yours. The first reason is that it can harm the user experience in a negative way, as ads can make your site look messy, slow down the loading times, or be intrusive, especially on mobile devices. The second reason is that revenue will most probably be very low unless you have a very high number of visitors, which makes this monetization strategy not very efficient for many. The third reason is that your control over the ads is quite limited and the ads may not be consistent with your brand or values. Moreover, google’s rigid rules might cause your account to be banned with minimum notice and no possibility of appealing. Lastly, using AdSense means that you will be dependent on a third party, therefore your power to create more sustainable income, such as product sales, memberships, or affiliate marketing that provides higher returns and more control, will be reduced.