Were they? If you have evidence where the seventy were consecrated as bishops please present it. But do recall that in Acts 1 Matthias was chosen from these other disciples and the Apostles still had to place hands on him to consecrate him as a bishop. Matthias was never an Apostle of Jesus because he was never called by Christ to be an apostle and was never sent by Christ. Only eleven were sent at the Great Commission. Paul would be called and sent later as recorded in Acts. Matthias was only a bishop and he establishes the scriptural proof for the Apostolic Succession of bishops.
An early work attributed to Hippolytus (but probably not actually his) lists them all as bishops. Some of them show up later in Acts of the Apostles. Here are their names and where they were bishops of: 1. James the Lord’s brother, bishop of Jerusalem 2. Cleopas, bishop of Jerusalem. 3. Matthias, who supplied the vacant place in the number of the twelve apostles. 4. Thaddeus, who conveyed the epistle to Augarus. 5. Ananias, who baptized Paul, and was bishop of Damascus. 6. Stephen, the first martyr. 7. Philip, who baptized the eunuch. 8. Prochorus, bishop of Nicomedia, who also was the first that departed, 11 believing together with his daughters. 9. Nicanor died when Stephen was martyred. 10. Timon, bishop of Bostra. 11. Parmenas, bishop of Soli. 12. Nicolaus, bishop of Samaria. 13. Barnabas, bishop of Milan. 14. Mark the evangelist, bishop of Alexandria. 15. Luke the evangelist. These two belonged to the seventy disciples who were scattered by the offence of the word which Christ spoke, “Except a man eat my flesh, and drink my blood, he is not worthy of me.” But the one being induced to return to the Lord by Peter’s instrumentality, and the other by Paul’s, they were honored to preach that Gospel on account of which they also suffered martyrdom, the one being burned, and the other being crucified on an olive tree. 16. Silas, bishop of Corinth. 17. Silvanus, bishop of Thessalonica. 18. Crisces (Crescens), bishop of Carchedon in Gaul. 19. Epænetus, bishop of Carthage. 20. Andronicus, bishop of Pannonia. 21. Amplias, bishop of Odyssus. 22. Urban, bishop of Macedonia. 23. Stachys, bishop of Byzantium. 24. Barnabas, bishop of Heraclea 25. Phygellus, bishop of Ephesus. He was of the party also of Simon. 26. Hermogenes. He, too, was of the same mind with the former. 27. Demas, who also became a priest of idols. 28. Apelles, bishop of Smyrna. 29. Aristobulus, bishop of Britain. 30. Narcissus, bishop of Athens. 31. Herodion, bishop of Tarsus. 32. Agabus the prophet. 33. Rufus, bishop of Thebes. 34. Asyncritus, bishop of Hyrcania. 35. Phlegon, bishop of Marathon. 36. Hermes, bishop of Dalmatia. 37. Patrobulus,1 bishop of Puteoli. 38. Hermas, bishop of Philippi. 39. Linus, bishop of Rome. 40. Caius, bishop of Ephesus. 41. Philologus, bishop of Sinope 42, 43. Olympus and Rhodion were martyred in Rome. 44. Lucius, bishop of Laodicea in Syria. 45. Jason, bishop of Tarsus. 46. Sosipater, bishop of Iconium 47. Tertius, bishop of Iconium. 48. Erastus, bishop of Panellas. 49. Quartus, bishop of Berytus. 50. Apollo, bishop of Cæsarea. 51. Cephas. 52. Sosthenes, bishop of Colophonia. 53. Tychicus, bishop of Colophonia. 54. Epaphroditus, bishop of Andriace. 55. Cæsar, bishop of Dyrrachium. 56. Mark, cousin to Barnabas, bishop of Apollonia. 57. Justus, bishop of Eleutheropolis. 58. Artemas, bishop of Lystra. 59. Clement, bishop of Sardinia. 60. Onesiphorus, bishop of Corone. 61. Tychicus, bishop of Chalcedon. 62. Carpus, bishop of Berytus in Thrace. 63. Evodus, bishop of Antioch. 64. Aristarchus, bishop of Apamea. 65. Mark, who is also John, bishop of Bibloupolis. 66. Zenas, bishop of Diospolis. 67. Philemon, bishop of Gaza. 68, 69. Aristarchus and Pudes. 70. Trophimus, who was martyred along with Paul. Jesus gave them the authority to teach in His name in Luke 10. I don’t know if they were all considered “bishops” at that point or if they were ordained later or if some were already ordained or not, but in my opinion it looks like Jesus’ commission to them was itself all they needed to be bishops. I don’t know if Jesus laid His hands on the Apostles either; traditionally we believe they were commissioned when He said “Do this in memory of me” and again when He breathed on them and said “Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained.” So Jesus’ method of commissioning His bishops was not necessarily by the laying on of hands. That was the Apostolic method and the method He wants His Church to use.
Also, re: Mattathias, it is not necessarily true that in Acts 1 they laid hands on him
in order to ordain him as a bishop. Sometimes they did that just to bless someone, and since the same action is involved in the Sacrament of Confirmation, we know for a fact that ordaining is not its only purpose.