The caffeine in tea varies wildly depending on the processing, region, harvest, etc. Coffee is similar but of course on average it's something like 10x more.
When you drink tea you re-steep until the leaves lose flavor, and the other chemicals reduce in potency similarly. Coffee you re-grind for every brew.
Also, the people that I know who drink tea, tend to drink it by the pot, whereas most coffee drinkers I know tend to drink by the cup.