Term limits for presidents prevent entrenchment of power. But congress (which has the most impact on laws that affect the daily lives of American citizens) has no limits on the length of time that members can serve.
We currently have 11 officials who have served in either the house or senate for more than 35 years. In the country's history, more than 100 individuals have served for longer than 35 years.
Any law that limits congressional terms would have to be proposed and accepted by those congress-people, and could be overturned at any later time by the next session of congress. So an amendment to the constitution is the only viable way to make term limits stick.
By voicing that term limits on congress are needed, action can begin at the state level toward a constitutional amendment.