The title of Arihant is given to the One who exists in a living form. The One who is Siddha is without a body and has attained nirvana or final liberation or moksha. These Siddhas cannot be called Arihants after this.
The twenty-four Tirthankaras on Bharat Kshetra were indeed Arihants when they were living, but they have now attained nirvana (the ultimate liberation of an enlightened being), they have attained the final moksha and have now become Siddhas and consequently they fall in the category of Siddhanum. People still continue to think of them as Arihants. They do not know that these Tirthankaras have become Siddhas. Simandhar Swami is the Arihant today. Only those who are currently present and living can be called 'Arihants'.
Before His departure, Lord Mahavir, the last of the twenty-four Tirthankaras, had said that the cycle of twenty-four Tirthankaras on Earth was ending and therefore there would be no more Tirthankaras on this planet for this time cycle. Lord Mahavir, asked people to worship the living Tirthankaras in Mahavideh Kshetra, because there are alive there!
Lord Mahavir had revealed everything! He knew that there would be no more Arihants on Earth, so whom would people worship? He therefore made public, the knowledge of the presence of Lord Simandhar Swami and nineteen other Tirthankaras in Mahavideh Kshetra. Once this information was divulged, it opened up this path of liberation. Acharya Kundakunda also revived this message a few centuries ago.
You can reap rewards if you worship the Arihants. Say your obedience in this way: 'I bow to the Arihant, whoever and wherever He may be in this entire universe.' This will yield wonderful results.