Zurich is considerably smaller than Amsterdam but you can see from the map linked above that regional buses don't go anywhere near the centre. The nearest they get is Bellevue stop which is 1.4 km from the central station. There is just one city bus route to the central station.
As someone who spent the first 16 years of his life or so in Zurich: "Bellevue" is very much what people in Zurich would consider part of the centre. It may be in one corner (roughly South East) of the centre, and the central station is in another corner (roughly North West) - but you also wouldn't say that a bus in London doesn't go "anywhere near the centre" because it stops at, say, Trafalgar Square and doesn't go on to King's Cross.
Zurich's main station - which is a more apt translation than central station anyway - is not really a good reference point for being the "centre" (speaking in terms of where people want to be as opposed to strictly geographic terms).
Zurich is fairly multi-nodal/multi-centred as cities of its size go, e.g. for many people, the "Stadelhofen" train station or the "Hardbrücke" train station are just as central for their needs as the main station. This is not comparable to buses stopping at a regional hub somewhere miles out of the city centre à la (apparently) Amsterdam.
I know the regional buses which you refer to, which stop at Bellevue, quite well as I used to take them as a high school student daily - they come in from the the right/Eastern shore of Lake Zurich, and would be totally lost in traffic if they kept going to the main station. Anyone who wants to go to the main station for a connecting train would be much better of taking the S-Bahn, either from their village or from the Tiefenbrunnen interchange. Anyone who wants to go those parts of the centre nearer the main station would be much quicker by either doing the same or, if more convenient for them, take the bus to Bellevue and then a tram onwards which is less bothered by traffic.
Generally, road-based traffic around Zurich central station is a pain at the best of times. It makes fairly little sense to send buses there except for the handful of urban buses (31, 46 from memory) that have to go past it anyway. You're much better off on a tram once you're inside the core.