why is tefilah a set text when it's meant to be a personal thing?

This is a great question that many have wondered about over the centuries! There are a few different approaches to this issue. Here are some:

1. R Y Halevi- Kuzari 3:19 writes:
“Communal prayer has many advantages. In the first instance a community will never pray for anything which is harmful to the individual, while the latter sometimes prays for something to the disadvantage of other individuals, and those individuals may pray for something that is to his disadvantage…A person who prays only for himself may be compared to someone who retires alone into his house, refusing to assist his fellow citizens in the repair of their walls. His expenditure is as great as his risk. The one, however, who joins the majority spends little, yet remains in safety.”

2. On the other hand, we have the Rambam. According to him (Mishna Torah Hil' Tefilah 1:4) spontaneous prayer was ideal. But, b/c of historical circumstances—no one was able to express himself in one language without making mistakes—we switched to fixed text. In fact, if you look in Tanach we see many examples of spontaneous Tefillah! One example is Channah, and we even learn a number of halakhot about davening from her!! (see Gem Brachot)

Given the reality that we have switched to a fixed text, the question that we have to address is: is there room for spontaneity??
It seems that not only is there room, but Chazal aware of the dangers of fixed text and actually MANDATE spontaneity! The Mishna in Masechet Avot says:

משנה מסכת אבות פרק ב משנה יג
רבי שמעון אומר הוי זהיר בקרית שמע ובתפלה וכשאתה מתפלל אל תעש תפלתך קבע אלא רחמים ותחנונים לפני המקום ברוך הוא שנאמר (יואל ב') כי אל חנון ורחום הוא ארך אפים ורב חסד ונחם על הרעה ואל תהי רשע בפני עצמך:

We can do this by inserting special tefillot within any of the brachot of bakash, we can add our own tefillah before "elokei nitzor."

Alternatively, we can look within Shmoneh Esreh and see that perhaps it is a "catch all" and we can find within it the words that we want to say if we look deeply enough.