Most terminal emulators date back to a time when one byte of input occupied one character-cell on-screen. Since Unicode arrived with combining characters, terminals have adapted to N bytes of input occupying one character-cell, or under special circumstances (Chinese characters, emoji) two character-cells.
Proper ligature support requires terminals to support N bytes of input mapping to M character cells... and unlike the N:1 or N:2 cases, the N:M case doesn't allow a hard-coded list of how many cells an input string covers, it varies from font to font. And that risks software designed for one font breaking in a terminal that uses a different font, if it has a different set of ligatures, or the ligatures are different sizes.
Proper ligature support requires terminals to support N bytes of input mapping to M character cells... and unlike the N:1 or N:2 cases, the N:M case doesn't allow a hard-coded list of how many cells an input string covers, it varies from font to font. And that risks software designed for one font breaking in a terminal that uses a different font, if it has a different set of ligatures, or the ligatures are different sizes.