Why is the Vbe threshold of a Si NPN transistor 0.7V and not -0.7V?
Short summary,

A diode can be forward or reversed biased. This concept only makes sense in the presence of external voltage bias.
The intrinsic potential difference of the p-n junction is -0.7V. The negative sign is by convention as it goes against turning on/forward biasing the diode.
By applying an external bias of +0.7V we counteract the intrinsic potential difference. The positive sign of the bias is by convention as it helps turning on the diode.

In detail,

p-n junction diodes are one of the most important Silicon building blocks, so they come with their own set of conventions:


When a diode is externally biased by applying a more POSITIVE potential to the p end of the junction than to the n end of the
junction, we say that it is FORWARD biased and we define that bias
voltage with a POSITIVE sign.
When a diode is externally biased by applying a more NEGATIVE potential to the p end of the junction than to the n end of the
junction, we say that it is REVERSED biased and we define that bias
voltage with a NEGATIVE sign.
Following the previous, we say than the forward current is POSITIVE and the reverse current is NEGATIVE. A positive current is
defined as the current in which the holes (h+) move in the same sense
and/or when the electrons (e-) move in the opposite sence.


You can think/visualize the p-n junction diode as having an internal or intrinsic potential difference of -0.7V (referenced from p to n). This will help you when analyzing any circuit!
Without external biasing, you can think/visualize the p-n junction as being &quot;intrinsecally reversed&quot;.
Now, if you overcome the intrinsic potential difference with an external positive voltage bias higher that 0.7V, you will have reduced to zero or reversed the intrinsic potential difference, effectively turning on the diode and making it forward biased.


Note that, in electronics, the way voltage and current signs are defined is simply a convention,

It's key to internalize these conventions as soon as possible during your learning process, as most people in the field, textbooks and articles adhere to these conventions.
It is a sign of deeper interest/curiosity/knowledge trying to understand the rationale behing some of these conventions. It'll surely will help you to step on more advanced concepts in the future. However, note that some of these conventions have no deep rationale behind, instead are a mixture of historical reasons or, in some cases, were defined arbitrarily.
Communication is a process inherent to human beings so, unless you want to avoid communication with others I suggest you to follow these sort of pre-established &quot;rules&quot;.