Because - as Double_A already said above - it’s just a “proprietary” service your broker offers.
They’ll only sell full shares at the exchange, not fractional shares.
At least two brokers I’ve been using split the sale order between full and fractional shares. Both let me (only) sell all my full shares with an online trading order - yet both retained the remaining fractional shares on my account. One made me print out, sign and e-mail them a separate instruction to dispose of the fractional share - at a separate transaction date and different price. The other, I just relinquished them altogether when closing the account before the end of the year. I’m sure I didn’t have to but it would have delayed things otherwise.