Some people were/are boosted, especially due to dynamic last season.
Also, theres big changes every season, so occasionally people climb pretty far due to the meta favouring their playstyle/champs without actually being very well rounded.
The reset ensures everyone has a chance to prove themselves in the current meta, and that ones ranked achievements for a season are for that season. If there was no reset (or if it even skewed upwards for reasons unclear to me), people could just get a high rank in a season favourable to them (or get boosted), and then play like 1 ranked match a month and still appear to be diamond, despite having the game knowledge and skill of a silver.