Why are boron compounds usually covalently bonded instead of containing B3+ ions?

Boron compounds are consistently covalently bonded rather than ionic because the energy required to remove three electrons from a boron atom to form a $B^{3+}$ ion is prohibitively high. Even if such an ion were formed, it would be too unstable to exist in a hydrated state, rendering the $B^{3+}(aq)$ ion unknown.12

Instead of forming ions, boron atoms undergo hybridization, where electrons are promoted from the $2s$ orbital to the $2p$ orbital, resulting in an outer electron configuration of $2s^{1}2p^2$. This process requires significantly less energy than complete ionization and allows boron to form stable covalent bonds through $s{p}^2$ or $s{p}^3$ hybrid configurations. These configurations enable boron to be three- or four-coordinated, respectively, facilitating the formation of a wide variety of molecular structures.12