Asians were trading with each other. Indians, Malay, Javans and Chinese were competing with each other for trade route supremacy. As South as Timor were in touch with Asian civilizations. How did no Asian explorers reach Australia before the Europeans?
The people living along the northern coastline of Australia, in the Kimberley, Arnhem Land, Gulf of Carpentaria and Cape York had encounters with various visitors for many thousands of years. People and traded goods moved freely between Australia and New Guinea.
Indonesian "Bajau" fishermen from the Spice Islands (e.g. Banda) have fished off the coast of Australia for hundreds of years. Macassan traders from Sulawesi regularly visited the coast of northern Australia to fish for trepang, an edible sea cucumber to trade with the Chinese since at least the early 18th century. Tamil sea-farers also had knowledge of Australia and Polynesia before European contact.
There was a high degree of cultural exchange, evidenced in Aboriginal rock and bark paintings, the introduction of technologies such as dug-out canoes and items such as tobacco and tobacco pipes, Macassan words in Aboriginal languages (e.g. Balanda for white person), and descendants of Malay people in Australian Aboriginal communities and vice versa, as a result of intermarriage and migration.
The myths of the people of Arnhem Land have preserved accounts of the trepang-catching, rice-growing Baijini people, who, according to the myths, were in Australia in the earliest times, before the Macassans. The Baijini have been variously interpreted by modern researchers as a different group of presumably South East Asian people, such as Bajau visitors to Australia who may have visited Arnhem Land before the Macassans, as a mythological reflection of the experiences of some Yolŋu people who have travelled to Sulawesi with the Macassans and came back, or, in more fringe views, even as visitors from China.
In 1944, a small number of copper coins with Arabic inscriptions were discovered on a beach in Jensen Bay on Marchinbar Island, part of the Wessel Islands of the Northern Territory. These coins were later identified as from the Kilwa Sultanate of east Africa.
So it is not true that Europeans were the first to discover Australia.
According to Wikipedia:
The ancestors of present-day Aboriginal Australians migrated from Asia by sea during the Pleistocene era.
This has been established mainly by ongoing genetic studies.
Why didn't Asians discover Australia first?
They did. The Aboriginals were originally asians. They discovered Australia first and settled there.