First, a terabyte is obviously quite a bit more than 64GB, and it will take up precious space inside the phone. If we can get 64GB on one ‘chip’ (which is a small portion of the microSD card package) you’d probably need 16 of those in your phone for a terabyte. A terabyte of memory will also require a somewhat more complex memory controller chip.
Second, that much memory, and the fancier memory controller, will eat up more power than the smaller amount does. There are ways to manage power by turning off sections that are less used or turning them to standby, but you could easily be randomly accessing stuff from several sections at once which would keep all of those sections active.
Both of those objections will likely be eroded within a few years by the typical advances in memory density and power consumption but using today’s memory chips it’s just not that practical.
A good example of using lots of high-density memory chips ganged together are the new SSD (solid state drive) that are an alternative to hard drives. You can get a ½ terabyte in a format that fits in a laptop computer, but notice that that drive size is at least half the size of a normal mobile phone.
…I’d also conjecture that not that many people want to have a terabyte of stuff on their phones either. They’d rather have stuff up in ‘the cloud’ or their home systems than have that much stuff on a device they could lose or is at much more risk of failure due to rough handling.