Instead of managing a simple cable, you're managing a cable with a massive brick in the middle that either takes up desk space or sits on the floor, getting kicked, trodden on, crushed between the desk and the wall, or worse hanging in midair, putting strain on the captive cable (and Apple have a dismal record on such things breaking and fraying). Instead of a standard figure 8/cloverleaf/kettle connector that can easily be replaced, you get a proprietary connector.
The M1 Air, 13" MBP, and Mini all just took existing products designed for Intel chips and converted them to M1 with the bare minimum of changes. That was (a) relatively quick and cheap and (b) kept the focus on the M1 itself rather than superficial design changes (...and avoided any risk of a new butterfly keyboard/flexgate/etc. spoiling the debut of the M1). Basically: mission accomplished, they've shown what Apple Silicon can do in a familiar product. The next wave is going to involve re-designing products to really take advantage of the M1 in terms of size, low power and cool running - the iMac being the first result.
I don't think any of the first wave of M1 Macs are likely to be around this time next year.
Because they wanted to make the new iMac incredibly thin. There isn't space for a power supply. It's not even deep enough to accommodate the depth of an IEC mains plug or standard ethernet plug.
The magsafe connector on the MacBook Pro was there to break away if someone yanked the cable. The magnetic connector on the iMac is there for the opposite reason - it is too shallow (and the cable too heavy) to be held in by friction, so it uses magnets to stop the connector falling out.