Just because one of your cells is having a good day doesn't mean the rest of them will be. Cancer is exactly that, one cell having become effectively immortal and duplicating constantly, disrupting the function of your other cells until you die.
In case its not clear, in the context of telomerase cellular immortality doesn't mean that the cell lasts forever, it means that the cell can duplicate itself indefinitely.
You can put a bunch of your cells in a petri dish and grow them indefinitely by activating the telomerase gene, but do the same in your body and you will just have removed one of the blocks to cancer.