Before the invention of printing, the ‘look’ of a script was determined by those in charge of producing large amounts of text—the professional scribes.
When Judeans first adopted the Assyrian version of the Canaanite script as their new script, some time in the 6th centuring BCE, the characters looked like this (pardon the letter order, which is left-to-right order, instead of right-to-left):
As you can see, there’s little rhyme or reason to them, and some of the characters—such as gimmel and dalet (green outline), vav and zayin (orange outline), and tzadi and quf and tav (blue outline)—looked precariously similar.
By the start of the Common Era, the characters had become more distinct:
(Source: Font 4Q418)
There were still some similarities to trip up the uninitiated (green, orange, blue, and purple outlines), but at least there was a broad, triangular-like pattern to the formation of the characters.
To resolve these confusing similarities, over the following centuries, these forms were refined and developed: in particular, scribes found that if they settled on a square template, with strong horizontal strokes, and just the occasional diagonal (e.g. in the letters aleph א, lamed ל, tzadi צ, and shin ש), they could more easily distinguish between the letters, and create a pleasing rhythm to them, as exemplified in this example from a 10th-century (CE) Hebrew Bible manuscript known as the Aleppo Codex:
(Source: National Library of Israel (id=990001147860205171)
The legibility and aesthetics of this calligraphy was so compelling, it became the new template across the Jewish world, culminating in an overtly square template (which is why it is known as ktav merubá כתב מרובע – ‘Square Script’):
(Source: File:Frank-ruehl.png – Wikimedia Commons)
But it’s important to note that the square template only applies to printed Hebrew of the Hebrew Bible, Talmud, and other traditional texts. Cursive Hebrew varied widely among Diaspora Jewry over the centuries, often echoing the design of the local Gentile script. Thus, Sephardi Jews in the Arab empire, wrote in a cursive with long strokes, somewhat like Arabic:
In the modern era, a variety of other typefaces have been developed, which are not necessarily square, and the modern cursive is altogether rounded, not unlike mirror forms of Roman-script italics:
This is because of the shared origin of the two alphabets, and chirodynamics, i.e. the natural tendencies of the (right) writing hand, as explained in my book, Aleph Through the Looking Glass.