Because the output of the IFFT operation is a vector of complex numbers. An RF signal is a real thing, so you can't just multiply a sine wave by a complex number and get something sensible.

However, you can map a complex number onto an RF carrier by multiplying the real part by $\cos \omega t$, and the imaginary part by $\sin \omega t$. The result is not complex -- it's all real. But the resulting arithmetic that you do on it, at the transmit and the receive end, is the same as if it were a complex number.

That's the beauty and convenience of I/Q modulation and demodulation.

Because the output of the IFFT operation is a vector of complex numbers. An RF signal is a real thing, so you can't just multiply a sine wave by a complex number and get something sensible.

However, you can map a complex number onto an RF carrier by multiplying the real part by $\cos \omega t$, and the imaginary part by $\sin \omega t$. This is I/Q modulation. The result is not complex -- it's all real. But the resulting arithmetic that you do on it, at the transmit and the receive end, is the same as if it were a complex number.

That's the beauty and convenience of I/Q modulation and demodulation.