Hard CC and assists

I've always enjoyed playing tanky heroes so after I heard that tanks were viable again I returned to play a little. I found that I often do my job as the tanky guy on the team but I get so locked down by hard CC that I don't get late damage off to get assists.

Recent example was me mid vs. 4 enemies, I know my team is coming so I lock the enemies down, they proceed to blow EVERYTHING they have (ults, summoners and items) on me and I die without being able to move or do anything. A second later in comes 3 of my team members and swoop up the now totally spent enemies. I don't get any kills or assists from that. I get death as a reward for serving up 4 easy kills for my team and I fall behind because I get no gold. So i'm ending winning games with nothing to show that I had a huge impact in team fights.

There are plenty of ways to get assists without doing damage, why is the above scenario accepted by Riot? Is the assist timer simply too short?