Knights and suchlike were for the most part part-timers too. After all, they went back to adminstering their estates after the campaign, and if their feudally-obligated time to serve in the army of their superior ran out and they for some reason didn't feel like sticking around were wholly within their legal rights to do so regardless of circumstances. Ditto for any other such troops serving for fixed periods out of legal obligation really. There's a recorded case from pre-Norman England of a bunch of Viking raiders besieged on some isle or cape getting away because the obligated time of the fyrd contignent bottling them in ran out before their replacements arrived - the guys simply picked up their things and walked home, leaving the raiders free to get away.

Mercenaries for their part had an unpleasant habit of becoming increasingly unreliable and uncontrollable if you started running low on funds to pay their wages with; they were certainly professional warriors, but not a standing army as such.

Full-time standing forces had their own limitations. One was that the full-time pros usually just formed the hard and reliable core of the army, but had to be bulked out by adding mercenaries, levies and suchlike to reach the sheer numbers and combat power required. Logistics, weather and seasons placed the ultimate limits. The Ottomans had a frightfully efficient commissariat, organization and supply system, but if their army was still stuck in siege camp around some Balkan town (or Vienna) when the snow fell they could essentially kiss both the campaign and better part of their troops' lives goodbye. Ability to keep the troops on the field doesn't really amount to much if you can no longer keep them alive...