Guilds are far less Chill than just begin casual PuGGer. Guilds expect you to live up to their standards and do all side content outside of raids, You can't just Login, raid, Log out.
Also Guild drama, People not signing up for raids, The Guild often rather go progress mythic than just do a weekly Heroic & Normal for loot.
Which often leads to many lootless nights and if you try to ask them to run atleast heroic before going mythic on a new reset they say "Just PuG it, we don't have time to do that"