Почему в википедии нет информации, когда существовали кельты?

Голубчик... Вы просто не умеете её читать.

The earliest archaeological culture that may justifiably be considered as Proto-Celtic is the Late Bronze Age Urnfield culture of central Europe from the last quarter of the second millennium BC. Their fully Celtic[2] descendants in central Europe were the people of the Iron Age Hallstatt culture (c. 800-450 BC) named for the rich grave finds in Hallstatt, Austria. By the later La Tène period (c. 450 BC up to the Roman conquest), this Celtic culture had expanded over a wide range of regions, whether by diffusion or migration: to the British Isles (Insular Celts), France and The Low Countries (Gauls), Bohemia, Poland and much of Central Europe, the Iberian Peninsula (Celtiberians, Celtici and Gallaeci) and northern Italy (Golaseccans and Cisalpine Gauls) and following the Gallic invasion of the Balkans in 279 BC as far east as central Anatolia (Galatians).
Evidence has been adduced for a Celtic language in the Tartessian inscriptions of south Portugal and southwest Spain (dating 7th-5th centuries BC).
The earliest directly attested examples of a Celtic language are the Lepontic inscriptions, beginning from the 6th century BC. Continental Celtic languages are attested only in inscriptions and place-names. Insular Celtic is attested from about the 4th century in ogham inscriptions, although it is clearly much earlier. Literary tradition begins with Old Irish from about the 8th century. Coherent texts of Early Irish literature, such as the Táin Bó Cúailnge (The Cattle Raid of Cooley), survive in 12th-century recensions.
By mid 1st millennium AD, following the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture and Insular Celtic had become restricted to Ireland, to the western and northern parts of Great Britain (Wales, Scotland, Cornwall and the Isle of Man), and to northern France (Brittany). Between the fifth and eighth centuries the Celtic-speaking communities of the Atlantic regions had emerged as a reasonably cohesive cultural entity. In language, religion, and art they shared a common heritage that distinguished them from the culture of surrounding polities. The Continental Celtic languages ceased to be widely used by the 6th century.

Вполне подробная датировка.. .И учтите к тому же, что "кельты" - это обобщающее название. Ну примерно как мы говорим "индейцы", хотя ацтеки, инки, майя, гуроны, делаверы, апачи, сиу.. .-это всё РАЗНЫЕ народы, во многом с разной историей. У кельтов примерно та же фигня.

Кельты - это не один народ.
Это огромное количество различных племен, которые в разное время заняли разные территории в Европе и Азии.

См. - Список кельтских племён
См. - Кельтские племена

Далее разбираемся с каждым племенем или группой племен отдельно - где? когда? и с кем? ))

Ты не поверишь - про славян та же история. И про германцев. И про греков. И про русских. Может, потому, что появились все до летописей и хроник, а живы до сих пор?

Потому что эту информацию еще никто не добавил. Стань первым!