Full HP Tahm Kench

I need someone to explain the logic behind that champion.So recently ( like past 2 week ) i started playing with/against a lot of tahm kench players that just build full hp.I played against him last game too,as toplaner.Since i knew he was going to do that meta full hp build i just tried to counter him with max hp items and my champ gnar's %hp damage, it didnt work.Even tho i got first kill and gain a level-gold advantadge in the lane,when he finished his warmog the game just finished for me.He just started chasing me everywhere.I know some people will say dodge his q he is so immobile , yes i dodged like %70 of them but every time i ate that slow i end up dying in his w because you know what ? with flat hp items he did 400 damage to me with this devour.That doesnt make any kind of sense at all.Of course there where times he did mistakes and i was really close to killing him , thanks to riot he just opened his 6 seconds 2k hp shield and walked away.