Some resorts include everything, others sell it as an option
At resorts in Thailand, "all-inclusive" does not always mean the same thing. Some only sell all-inclusive stays. Others offer room-only, breakfast or half-board rates, with all-inclusive as one option among several.
All-inclusive as one rate among several
Robinson Khao Lak, Hyatt Regency Koh Samui, Dusit Thani Krabi Beach Resort, Outrigger Khao Lak Beach Resort, Centara Villas Samui, Hyatt Regency Phuket Resort, JW Marriott Khao Lak Resort & Spa, Avani+ Hua Hin Resort, Centara Karon Resort Phuket. When you book one of these, pick the rate marked all-inclusive. The cheapest price shown is often room only or half board.
Before you book
The Andaman coast (Phuket, Khao Lak, Krabi) is best from November to April, and the Gulf (Koh Samui) is drier from January to August.